Web Performance with Androidrsquos Battery-Saver ModeUtkarsh Goel

Akamai Technologies Incugoelakamaicom

Stephen LudinAkamai Technologies Inc

sludinakamaicom

Moritz SteinerAkamai Technologies Incmosteineakamaicom

ABSTRACTA Web browser utilizes a devicersquos CPU to parse HTMLbuild a Document Object Model a Cascading Style SheetsObject Model and render trees and parse compile andexecute computationally-heavy JavaScript A powerful CPUis required to perform these tasks as quickly as possible andprovide the user with a great experience However increasedCPU performance comes with increased power consumptionand reduced battery life on mobile devices As an optionto extend battery life Android offers a battery-saver modethat when activated turns off the power-hungry and fasterprocessor cores and turns on the battery-conserving andslower processor cores on the device The transition fromusing faster processor cores to using slower processor coresthrottles the CPU clock speed on the device and thereforeimpacts the webpage load processWe utilize a large-scale data-set collected by a real user

monitoring system of a major content delivery networkto investigate the impact of Androidrsquos battery-saver modeon various mobile Web performance metrics Our analysissuggests that users of select smartphones of Huawei andSony experience a sudden or gradual degradation in Webperformance when battery-saver mode is active Battery-saver mode on newer flagship smartphones howeverdoes not impact the mobile Web performance Finally weencourage for new website design goals that treat slow (andthrottled-CPU) devices kindly in favor of improvingend-user experience and suggest that Web performancemeasurements should be aware of user device battery chargelevels to correctly associate Web performance

1 INTRODUCTIONIn the last several years mobile websites have growndrastically in both complexity and size [32 33] This growthhas led to slower page loads and higher user frustration withthe website [3] As websites continue to grow in complexitythe key to improve website responsiveness is to build fasternetworks optimize website content and produce mobiledevices with faster CPUs While many networks havealready begun to deploy new infrastructure and supportfaster communication protocols [28 34 38 43] and mostwebsites already employ a suite of website optimizationtechniques [35] the mobile device CPU remains a limiting

factor to mobile Web performance [50] Unlike desktop andlaptop CPUs Mobile CPUs are designed for power efficiencyMore specifically Android smartphones have a battery-

saver mode that lowers the battery consumption when thecharge level drops below a certain threshold Among otherthings the battery-saver mode reduces the devicersquos perfor-mance by throttling the CPU clock speed where it deac-tivates the power-hungry and faster processor cores andactivates the battery-saving and slower processor cores [36]The process of loading Web pages includes HTML parsingdownloading and processing of JavaScript Cascading StyleSheets (CSS) image resources executing computationally-heavy JavaScript and building the Document Object Model(DOM) CSS Object Model (CSSOM) and render tree [37]Since all these tasks make use of the devicersquos CPU resourcesin this paper we seek to investigate whether or not a throt-tled CPU clock speed under an active battery-saver mode de-grades the mobile Web performance for the end-user Simplyput domobile websites load slower on Android phones whenthe battery charge levels drop below a certain thresholdSince the impact of battery-saver mode on mobile Web

performance has not garnered much developer interest in thepast our goal with this paper is to bring awareness to the de-veloper community about potential performance impacts andthus motivate the need for new website design goals and de-cisions that treat mobile devices differently especially whenbattery-saver modes are active and the CPU clock speeds arethrottled We make the following contributions in the paper

Dataset Richness To investigate the impact of batterysaver modes on mobile Web performance we utilized alarge-scale Web performance dataset collected by AkamaimPulse for websites loaded by real users on various mobiledevices [13] Our dataset contains various Web performancemetrics collected for 10 million pages loaded on 300 differ-ent smartphone models connected to 81 cellular ISPs in 39countries from July 2017 to March 2018

Inferences Drawn Using a large-scale mobile Web perfor-mance dataset we discover that under battery-saver modeselect phones from Huawei Sony Xperia and SamsungGalaxy series degrademobileWeb performancemetrics suchas the page load time (PLT) time to first paint (TTFP) totalLongTask time time to interactive (TTI) and frame rate [26 19 27] The data also suggest that the battery-saver modemakes a higher impact onWeb performance whenWeb pages

load in faster mobile network conditions The above findingsdemonstrate a clear need for new website design goals thatwould go hand-in-hand with the understanding of how userdevices with low battery charge levels could degrade themobile Web experience Specifically developers might wantto build websites that adapt to different battery situations toovercome any user-perceived responsiveness issues inflictedby the battery-saver mode Additionally to reduce utilizationof the throttled CPU users may use browsers that offloadCPU-intensive computations to the cloud [25 29 30]The rest of the paper is organized as follows Section 2

gives a background on Androidrsquos battery-saver modes InSections 3 and 4 we discuss our data collection methodologyand the impact of battery-saver mode on various Webperformance metrics In Section 5 we discuss the relatedwork Finally we conclude in Section 6

2 BACKGROUNDThe battery-saver mode when activated reduces the screenbrightness limits the use of WiFi disables power-consuminglocation-sharing services and reduces the applicationbackground activity Android smartphones such as LGG5 Huawei Y6 Elite Alcatel Pixi 4 and many othersprovide two user-configurable options for the battery chargelevel threshold at which the battery-saver mode turns onautomatically [15 16 18 47] These threshold values are 5and 15 However other Android smartphones such LGG3 and LG VS985 provide four user-configurable optionsfor this threshold which are 10 20 30 and 50 [31]Similarly Samsung phones such as Galaxy S6 Galaxy J5and others also provide four user-configurable options forthis threshold which are 5 10 20 and 50 [10 20] Notethat even though there is an automated way to turn on thebattery-saver mode every time the battery charge level dropsbelow a certain threshold the activation of this feature on thedevice depends on a userrsquos interest in saving battery chargeOther smartphones such as Samsung Galaxy S8 Note

8 and Galaxy S5 do not provide a way to set a thresholdwhich means that the activation and deactivation of thepower-saving modes must be done manually by users everytime they want to save power [12 17] The activation ofthe power-saving modes on these devices may even belower compared to the devices that offer a threshold forautomatically activating the power-saving modeSonyrsquos Xperia Z5 Compact has several power-saving

modes such as the Doze mode that turns on automaticallyto save power when the device screen is turned off [14]Other power-saving modes such as the Stamina modeallow users to set a battery charge level threshold at whichpower-saving mode activates Moreover the power-savingmode on Sony Xperia Z5 Compact also allows users to

decide whether or not the CPU clock speed should bethrottled in addition to or instead of disabling mobile dataand WiFi [22] As such some users may choose to activatepower-saving mode without throttling the CPU clock speedwhile others may choose to throttle the CPU clock speed

3 DATA COLLECTION METHODOLOGYTo analyze the different performance metrics pertaining towebsites loaded on various real user smartphones we utilizedthe data collected by Akamairsquos mPulse product [13] mPulseembeds a lightweight JavaScript snippet to some HTML re-sponses and leverages the Web browser-exposed NavigationTiming API to collect performance-related information toestimate the time taken to load a page [2] mPulse also uti-lizes the browser exposed Battery Status API to associate themeasured website performance data with the devicersquos batterycharge level information at the time of measurement [5] Thecollected data is reported back to Akamai servers [44] whichwe analyze to assess the impact of battery saver modes onpage load performance Our data consists of a total of 10 mil-lion page load transactions for 480 unique websites loadedon 300 different smartphone models connected to 81 cellularISPs in 39 countries from July 2017 through March 2018To investigate the Web performance experienced on

devices with low battery charge levels we calculate themedian page load time (PLT) observed for page loadspertaining to 6-field buckets comprised of 1) the countryname in which the page was loaded 2) the ISP over whichthe page was loaded (note that the first two pieces ofinformation are deduced from the MaxMindrsquos databaseof mapping IP addresses to geographical locations [40])3) the URL of the website loaded 4) the smartphone modelused to load the website 5) the HTTP protocol version(HTTP11 or HTTP2) used and 6) the device batterypercentage ranging from 1 to 100 Note that we refer toPLT as the time from the start of page navigation until theloadEventStart event is triggered by the Web browser [2]Also note that bucketing the data with this approach helpsto precisely understand how fast a given website loads ona given ISP network under specific constraints such as theuserrsquos country userrsquos ISP website URL the smartphone andthe HTTP protocol and thus mitigate the influence of manyfactors that might affect the analysis of Web performanceIn addition to calculating the median PLT for beacons

in each bucket we calculate the 10th 25th 75th and 90thpercentile PLT values By calculating these percentile valuesfor each of the 6-field bucket we could assume that the10th percentile PLT tends to represent page loads in fastcellular networks and that the 90th percentile PLT tendsto represent page loads in slower cellular networks Thiscategorization would help us understand how the impact

of battery-saver mode changes as network performanceimproves or degradesFinally similarly to how we calculate PLT distributions

we also calculate the TTFP observed for loading differentwebsites on different smartphones Additionally we collectthe total LongTask time the time to interactive and theframe rate observed when loading websites under variousbattery charge levels Note that since the LongTask timeto interactive and frame rate metrics depend on the devicehardware as opposed to the network performance for thesemetrics we do not calculate 10th 25th 50th 75th and 90thpercentile values (because we represent percentile valuesas network speed) but instead plot the whole distributionsin appropriate figuresNote that the dataset we prepared consists of 25 unique

combinations (comprising of country ISP URL smartphonename and HTTP protocol) for which mPulse library was exe-cuted on at least 100 page loads for each of the battery chargelevels ranging from 1 to 100 - a total of at least 10000 bea-cons for each combination Additionally the dataset consistsof 63 unique combinations for which mPulse library was ex-ecuted on at least 100 page-load transactions for at least 90battery charge levels - a total of at least 9000 beacons for eachcombination The low number of combinations observed inthe dataset is a consequence of the fact that only about 25of the total page loads occurred when battery charge lev-els were below 15 which yielded less than 100 beaconsfor some battery charge levels on some combinations Addi-tionally the low number of page loads under battery chargelevels less than 15 may suggest that either most users keeptheir phones charged over 15 at all times or that when bat-tery charge levels drop below 15 users reduce their Webbrowsing activities in favor of conserving battery charge

4 MEASURINGWEBSITE PERFORMANCETo estimate the performance of a website we analyze sev-eral Web performance metrics under different device batterycharge levels such as PLT and TTFP ndash the time since the startof the page navigation until the browser paints the first pixelsWe also analyze the total LongTask time ndash the time duringwhich the browser mainUI thread is blocked and thereforethe user cannot interact with the page [24] Finally we mea-sure the average rate of printing frames on the screen [27]

41 Measuring Page Load Time (PLT)411 Performance on Huawei Y6 Elite In Figure 1 we showvarious percentile PLT values for a page loaded on HuaweiY6 Elite smartphone From the figure we can observe thatacross all percentiles the PLT inflates as soon as the batterycharge level drops to 15 likely due to degraded CPU clockspeeds triggered by the battery-saver mode The figure also

0 20 40 60 80 100

6000

10000

14000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 1 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei Y6 Elite mobile device

Table 1 Summary of PLT distributions on Huawei Y6Elite phone

suggests that on most Huawei Y6 Elite smartphones thedefault threshold for when the battery-saver mode initiatesis set to 15In the Table 1 we summarize the analysis from Figure 1

to compare the PLT observed when battery charge levelsare for example 8 and 50 Note that the battery chargelevels 8 and 50 are just two example representative pointsfor comparing the performance under the battery-saver andnormal modes

From the table we can observe that the relative differencein the page load decreases as the network performancedegrades Specifically among the two battery levels 8 and50 for page loads represented by the 10th percentile distri-bution (p10) the PLTs at 8 are higher by 28 compared tothe PLTs at 50 Whereas for the 90th percentile distribution(p90) the PLTs at 8 are higher by 15 compared to thePLTs at 50 This downward trend suggests that whenwebsites are loaded on faster networks the smartphonersquosCPU becomes a more significant bottleneck for websiteperformance Indeed this trend is similar to a previousresearch study that investigates how device performanceimpacts Web performance [50]

0 20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75

Figure 2 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei P8 Lite (2015) mobile device

Figure 3 Summary of PLT distributions onHuawei P8 Lite phone

Both PLT and TTFP metrics make direct impact on theuser experience As such an increase in these metrics dueto degraded CPU clock speeds represents a degradation inthe user experience

412 Performance on Huawei P8 Lite (2015 model) Similarlyto Figure 1 as shown in Figure 2 we observe a suddeninflation in PLTs when loading a Web page on the 2015model of Huawei P8 Lite smartphone Since the inflationoccurs when the battery charge level is at 10 it appears thatthe battery-saver mode on the Huawei P8 Lite smartphoneturns on for most users at 10Additionally as shown in Table 3 when we compare the

relative PLT differences across different distributions wecan observe that the impact of throttled CPU clock speedson PLT appears to be more significant when pages areloaded in fast network conditions

We observed similar trends in performance for other web-sites that loaded on Huawei P8 Lite smartphone howeverwe do not show them due to page limits Additionally notethat for websites loaded on the 2017 model of Huawei P8 Litesmartphone we did not observe sudden inflation in PLT orTTFP metrics at any battery charge level potentially becausethe 2017 model has a faster CPU that does not impact thepage load despite the drop in CPU performance [8] Finallywe noticed that newer smartphones from the same family

20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 4 Page load time distributions across differentdevice battery charge levels as measured for a pageloaded on Sony Xperia Z5 Compact mobile device

such as Huawei P9 Lite and P10 Lite also do not degradewebsite performance under low battery charge levels

413 Performance on Sony Xperia Z5 Compact In Figure 4we show that when pages load on Sony Xperia Z5 Compactsmartphone there exists an upward slope indicating thatpage load times continue to rise as battery charge levelsdrop below 40 We have not been able to identify the causefor such an upward slope as opposed to a sudden increasedin page load time even though this device allows users toset a threshold at which the battery-saver mode activatesPerhaps this device has a linear slowdown in the CPU clockspeed when the battery charge levels reach 40 Note that wedid not observe such a strong upward slope for other Sonydevices such as Xperia X Compact Xperia X Performanceand Xperia XZ and in fact there was no inflation in PLT forthese devices under low battery charge levels

414 Performance on High-end Phones We compare thewebsite performance for pages loaded on various new andold Samsung flagship smartphones under different batterycharge levels Note that for the purposes of comparingperformance across different Samsung devices in Figure 5we only show the PLT values calculated for the 50thpercentile distribution at various battery charge levelsAdditionally note that for making the comparison easierto understand in Figure 5 we compare the performanceobserved for Huawei Y6 Elite smartphone for the samewebsite as the one used in Figure 1

From Figure 5 we observe that when the website isloaded on newer devices with high-performance CPU thePLT gets lower across all battery charge levels For examplethe PLTs on Note 8 are about 4 seconds whereas the PLTson Galaxy S6 are about 55 seconds Similarly PLTs onGalaxy S5 are about 65 seconds and the PLTs on Y6 Eliteunder non-throttled CPU hardware are about 8 secondsThe performance differences across devices show that the

0 20 40 60 80 100

4000

6000

8000

10000

Battery Level ()

PLT

(ms)

Note 8Galaxy S7

Galaxy S6Galaxy S5

Y6 Elite

Figure 5 PLT distributions across different device bat-tery charge levels as measured for a page loaded onvarious Samsung smartphones and Huawei Y6 Elite

0 20 40 60 80 100

1000

3000

5000

Battery Level ()

TTFP

(ms)

p10 p25 p50 p75 p90

Figure 6 TTFP distributions across different devicebattery charge levels as measured for a page loadedon Huawei Y6 Elite mobile device

website performance can be improved by upgrading thedevice hardware regardless of the battery charge levelAdditionally from Figure 5 we observe that PLTs on

Note 8 do not experience sudden or gradual increase at anybattery charge level Since Samsung Note 8 does not providea default user configurable option to enable the battery-savermode when the battery charge level reaches a certain thresh-old and that users must manually activate the battery-savermode perhaps most users tend to not activate the battery-saver mode and therefore we do not observe any inflation inPLTs Alternatively since Galaxy Note 8 has octa-core pro-cessors built-in perhaps even when the battery-saver modeis active the throttled CPU clock speed is high enough tonot negatively impact the page load process or the PLT [11]However we acknowledge that a better understanding of theinner workings of the device would help bear this out Forother devices such as Galaxy S7 and S6 we do not observeany inflation in PLT regardless of the battery charge levelSimilarly to Note 8 perhaps the throttled CPU clock speedson these devices are high enough to not impact the PLT

1000

2000

3000

4000

5000

Battery Level ()

Tota

l Lon

gTas

k Ti

me

(ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 7 LongTask time distributions across differentdevice battery charge levels as measured for a pageloaded on P8 Lite (2015) smartphone

42 Measuring TTFPIn Figure 6 we show various percentile TTFP values forthe same page as Figure 1 when loaded on Huawei Y6 Elitesmartphone From the figure we can observe that thereexists a sudden inflation in the TTFP when the batterycharge levels drop below 15 This trend suggests that thetime when the browser paints the first pixels also increaseswhen the device enables the battery-saver mode Similarlyto Figure 1 we also noticed sudden inflation in TTFP atbattery charge level 15 when loading websites on the 2015model of Huawei P8 Lite smartphone

43 Measuring LongTask TimeLongTask is a relatively new Web performance measure-ment API that allows identification of resources that makewebsites unresponsive to user interactions [19] Morespecifically Web developers could use the LongTask API todetect the presence of tasks that block the browser UImainthread for at least 50 milliseconds When a website loadsresources that block the browser UImain thread the useris unable to interact with the page Specifically a long taskprevents the page from responding to user actions suchas scroll click tap key wheel etc until the long task hasfinished executing This is because when a long task isexecuting all user actions are queued behind the long taskPoorly designed JavaScript code is one example of what

might cause a browser main thread to block for over50ms [46] Since the current LongTask API (v1) does notreveal the URL of the long task (though the second versionof the API will provide such detail including the linenumber that caused the long task [45]) it is unclear as towhat particular resources block the browser main threadTherefore we only focus on investigating whether or notwe observe a rise in the total LongTask time when deviceCPU clock speed reduces when battery charge levels dropbelow a certain threshold as opposed to discussing the root

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

load in faster mobile network conditions The above findingsdemonstrate a clear need for new website design goals thatwould go hand-in-hand with the understanding of how userdevices with low battery charge levels could degrade themobile Web experience Specifically developers might wantto build websites that adapt to different battery situations toovercome any user-perceived responsiveness issues inflictedby the battery-saver mode Additionally to reduce utilizationof the throttled CPU users may use browsers that offloadCPU-intensive computations to the cloud [25 29 30]The rest of the paper is organized as follows Section 2

gives a background on Androidrsquos battery-saver modes InSections 3 and 4 we discuss our data collection methodologyand the impact of battery-saver mode on various Webperformance metrics In Section 5 we discuss the relatedwork Finally we conclude in Section 6

2 BACKGROUNDThe battery-saver mode when activated reduces the screenbrightness limits the use of WiFi disables power-consuminglocation-sharing services and reduces the applicationbackground activity Android smartphones such as LGG5 Huawei Y6 Elite Alcatel Pixi 4 and many othersprovide two user-configurable options for the battery chargelevel threshold at which the battery-saver mode turns onautomatically [15 16 18 47] These threshold values are 5and 15 However other Android smartphones such LGG3 and LG VS985 provide four user-configurable optionsfor this threshold which are 10 20 30 and 50 [31]Similarly Samsung phones such as Galaxy S6 Galaxy J5and others also provide four user-configurable options forthis threshold which are 5 10 20 and 50 [10 20] Notethat even though there is an automated way to turn on thebattery-saver mode every time the battery charge level dropsbelow a certain threshold the activation of this feature on thedevice depends on a userrsquos interest in saving battery chargeOther smartphones such as Samsung Galaxy S8 Note

8 and Galaxy S5 do not provide a way to set a thresholdwhich means that the activation and deactivation of thepower-saving modes must be done manually by users everytime they want to save power [12 17] The activation ofthe power-saving modes on these devices may even belower compared to the devices that offer a threshold forautomatically activating the power-saving modeSonyrsquos Xperia Z5 Compact has several power-saving

modes such as the Doze mode that turns on automaticallyto save power when the device screen is turned off [14]Other power-saving modes such as the Stamina modeallow users to set a battery charge level threshold at whichpower-saving mode activates Moreover the power-savingmode on Sony Xperia Z5 Compact also allows users to

decide whether or not the CPU clock speed should bethrottled in addition to or instead of disabling mobile dataand WiFi [22] As such some users may choose to activatepower-saving mode without throttling the CPU clock speedwhile others may choose to throttle the CPU clock speed

3 DATA COLLECTION METHODOLOGYTo analyze the different performance metrics pertaining towebsites loaded on various real user smartphones we utilizedthe data collected by Akamairsquos mPulse product [13] mPulseembeds a lightweight JavaScript snippet to some HTML re-sponses and leverages the Web browser-exposed NavigationTiming API to collect performance-related information toestimate the time taken to load a page [2] mPulse also uti-lizes the browser exposed Battery Status API to associate themeasured website performance data with the devicersquos batterycharge level information at the time of measurement [5] Thecollected data is reported back to Akamai servers [44] whichwe analyze to assess the impact of battery saver modes onpage load performance Our data consists of a total of 10 mil-lion page load transactions for 480 unique websites loadedon 300 different smartphone models connected to 81 cellularISPs in 39 countries from July 2017 through March 2018To investigate the Web performance experienced on

devices with low battery charge levels we calculate themedian page load time (PLT) observed for page loadspertaining to 6-field buckets comprised of 1) the countryname in which the page was loaded 2) the ISP over whichthe page was loaded (note that the first two pieces ofinformation are deduced from the MaxMindrsquos databaseof mapping IP addresses to geographical locations [40])3) the URL of the website loaded 4) the smartphone modelused to load the website 5) the HTTP protocol version(HTTP11 or HTTP2) used and 6) the device batterypercentage ranging from 1 to 100 Note that we refer toPLT as the time from the start of page navigation until theloadEventStart event is triggered by the Web browser [2]Also note that bucketing the data with this approach helpsto precisely understand how fast a given website loads ona given ISP network under specific constraints such as theuserrsquos country userrsquos ISP website URL the smartphone andthe HTTP protocol and thus mitigate the influence of manyfactors that might affect the analysis of Web performanceIn addition to calculating the median PLT for beacons

in each bucket we calculate the 10th 25th 75th and 90thpercentile PLT values By calculating these percentile valuesfor each of the 6-field bucket we could assume that the10th percentile PLT tends to represent page loads in fastcellular networks and that the 90th percentile PLT tendsto represent page loads in slower cellular networks Thiscategorization would help us understand how the impact

of battery-saver mode changes as network performanceimproves or degradesFinally similarly to how we calculate PLT distributions

we also calculate the TTFP observed for loading differentwebsites on different smartphones Additionally we collectthe total LongTask time the time to interactive and theframe rate observed when loading websites under variousbattery charge levels Note that since the LongTask timeto interactive and frame rate metrics depend on the devicehardware as opposed to the network performance for thesemetrics we do not calculate 10th 25th 50th 75th and 90thpercentile values (because we represent percentile valuesas network speed) but instead plot the whole distributionsin appropriate figuresNote that the dataset we prepared consists of 25 unique

combinations (comprising of country ISP URL smartphonename and HTTP protocol) for which mPulse library was exe-cuted on at least 100 page loads for each of the battery chargelevels ranging from 1 to 100 - a total of at least 10000 bea-cons for each combination Additionally the dataset consistsof 63 unique combinations for which mPulse library was ex-ecuted on at least 100 page-load transactions for at least 90battery charge levels - a total of at least 9000 beacons for eachcombination The low number of combinations observed inthe dataset is a consequence of the fact that only about 25of the total page loads occurred when battery charge lev-els were below 15 which yielded less than 100 beaconsfor some battery charge levels on some combinations Addi-tionally the low number of page loads under battery chargelevels less than 15 may suggest that either most users keeptheir phones charged over 15 at all times or that when bat-tery charge levels drop below 15 users reduce their Webbrowsing activities in favor of conserving battery charge

4 MEASURINGWEBSITE PERFORMANCETo estimate the performance of a website we analyze sev-eral Web performance metrics under different device batterycharge levels such as PLT and TTFP ndash the time since the startof the page navigation until the browser paints the first pixelsWe also analyze the total LongTask time ndash the time duringwhich the browser mainUI thread is blocked and thereforethe user cannot interact with the page [24] Finally we mea-sure the average rate of printing frames on the screen [27]

41 Measuring Page Load Time (PLT)411 Performance on Huawei Y6 Elite In Figure 1 we showvarious percentile PLT values for a page loaded on HuaweiY6 Elite smartphone From the figure we can observe thatacross all percentiles the PLT inflates as soon as the batterycharge level drops to 15 likely due to degraded CPU clockspeeds triggered by the battery-saver mode The figure also

0 20 40 60 80 100

6000

10000

14000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 1 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei Y6 Elite mobile device

Table 1 Summary of PLT distributions on Huawei Y6Elite phone

suggests that on most Huawei Y6 Elite smartphones thedefault threshold for when the battery-saver mode initiatesis set to 15In the Table 1 we summarize the analysis from Figure 1

to compare the PLT observed when battery charge levelsare for example 8 and 50 Note that the battery chargelevels 8 and 50 are just two example representative pointsfor comparing the performance under the battery-saver andnormal modes

From the table we can observe that the relative differencein the page load decreases as the network performancedegrades Specifically among the two battery levels 8 and50 for page loads represented by the 10th percentile distri-bution (p10) the PLTs at 8 are higher by 28 compared tothe PLTs at 50 Whereas for the 90th percentile distribution(p90) the PLTs at 8 are higher by 15 compared to thePLTs at 50 This downward trend suggests that whenwebsites are loaded on faster networks the smartphonersquosCPU becomes a more significant bottleneck for websiteperformance Indeed this trend is similar to a previousresearch study that investigates how device performanceimpacts Web performance [50]

0 20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75

Figure 2 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei P8 Lite (2015) mobile device

Figure 3 Summary of PLT distributions onHuawei P8 Lite phone

Both PLT and TTFP metrics make direct impact on theuser experience As such an increase in these metrics dueto degraded CPU clock speeds represents a degradation inthe user experience

412 Performance on Huawei P8 Lite (2015 model) Similarlyto Figure 1 as shown in Figure 2 we observe a suddeninflation in PLTs when loading a Web page on the 2015model of Huawei P8 Lite smartphone Since the inflationoccurs when the battery charge level is at 10 it appears thatthe battery-saver mode on the Huawei P8 Lite smartphoneturns on for most users at 10Additionally as shown in Table 3 when we compare the

relative PLT differences across different distributions wecan observe that the impact of throttled CPU clock speedson PLT appears to be more significant when pages areloaded in fast network conditions

We observed similar trends in performance for other web-sites that loaded on Huawei P8 Lite smartphone howeverwe do not show them due to page limits Additionally notethat for websites loaded on the 2017 model of Huawei P8 Litesmartphone we did not observe sudden inflation in PLT orTTFP metrics at any battery charge level potentially becausethe 2017 model has a faster CPU that does not impact thepage load despite the drop in CPU performance [8] Finallywe noticed that newer smartphones from the same family

20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 4 Page load time distributions across differentdevice battery charge levels as measured for a pageloaded on Sony Xperia Z5 Compact mobile device

such as Huawei P9 Lite and P10 Lite also do not degradewebsite performance under low battery charge levels

413 Performance on Sony Xperia Z5 Compact In Figure 4we show that when pages load on Sony Xperia Z5 Compactsmartphone there exists an upward slope indicating thatpage load times continue to rise as battery charge levelsdrop below 40 We have not been able to identify the causefor such an upward slope as opposed to a sudden increasedin page load time even though this device allows users toset a threshold at which the battery-saver mode activatesPerhaps this device has a linear slowdown in the CPU clockspeed when the battery charge levels reach 40 Note that wedid not observe such a strong upward slope for other Sonydevices such as Xperia X Compact Xperia X Performanceand Xperia XZ and in fact there was no inflation in PLT forthese devices under low battery charge levels

414 Performance on High-end Phones We compare thewebsite performance for pages loaded on various new andold Samsung flagship smartphones under different batterycharge levels Note that for the purposes of comparingperformance across different Samsung devices in Figure 5we only show the PLT values calculated for the 50thpercentile distribution at various battery charge levelsAdditionally note that for making the comparison easierto understand in Figure 5 we compare the performanceobserved for Huawei Y6 Elite smartphone for the samewebsite as the one used in Figure 1

From Figure 5 we observe that when the website isloaded on newer devices with high-performance CPU thePLT gets lower across all battery charge levels For examplethe PLTs on Note 8 are about 4 seconds whereas the PLTson Galaxy S6 are about 55 seconds Similarly PLTs onGalaxy S5 are about 65 seconds and the PLTs on Y6 Eliteunder non-throttled CPU hardware are about 8 secondsThe performance differences across devices show that the

0 20 40 60 80 100

4000

6000

8000

10000

Battery Level ()

PLT

(ms)

Note 8Galaxy S7

Galaxy S6Galaxy S5

Y6 Elite

Figure 5 PLT distributions across different device bat-tery charge levels as measured for a page loaded onvarious Samsung smartphones and Huawei Y6 Elite

0 20 40 60 80 100

1000

3000

5000

Battery Level ()

TTFP

(ms)

p10 p25 p50 p75 p90

Figure 6 TTFP distributions across different devicebattery charge levels as measured for a page loadedon Huawei Y6 Elite mobile device

website performance can be improved by upgrading thedevice hardware regardless of the battery charge levelAdditionally from Figure 5 we observe that PLTs on

Note 8 do not experience sudden or gradual increase at anybattery charge level Since Samsung Note 8 does not providea default user configurable option to enable the battery-savermode when the battery charge level reaches a certain thresh-old and that users must manually activate the battery-savermode perhaps most users tend to not activate the battery-saver mode and therefore we do not observe any inflation inPLTs Alternatively since Galaxy Note 8 has octa-core pro-cessors built-in perhaps even when the battery-saver modeis active the throttled CPU clock speed is high enough tonot negatively impact the page load process or the PLT [11]However we acknowledge that a better understanding of theinner workings of the device would help bear this out Forother devices such as Galaxy S7 and S6 we do not observeany inflation in PLT regardless of the battery charge levelSimilarly to Note 8 perhaps the throttled CPU clock speedson these devices are high enough to not impact the PLT

1000

2000

3000

4000

5000

Battery Level ()

Tota

l Lon

gTas

k Ti

me

(ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 7 LongTask time distributions across differentdevice battery charge levels as measured for a pageloaded on P8 Lite (2015) smartphone

42 Measuring TTFPIn Figure 6 we show various percentile TTFP values forthe same page as Figure 1 when loaded on Huawei Y6 Elitesmartphone From the figure we can observe that thereexists a sudden inflation in the TTFP when the batterycharge levels drop below 15 This trend suggests that thetime when the browser paints the first pixels also increaseswhen the device enables the battery-saver mode Similarlyto Figure 1 we also noticed sudden inflation in TTFP atbattery charge level 15 when loading websites on the 2015model of Huawei P8 Lite smartphone

43 Measuring LongTask TimeLongTask is a relatively new Web performance measure-ment API that allows identification of resources that makewebsites unresponsive to user interactions [19] Morespecifically Web developers could use the LongTask API todetect the presence of tasks that block the browser UImainthread for at least 50 milliseconds When a website loadsresources that block the browser UImain thread the useris unable to interact with the page Specifically a long taskprevents the page from responding to user actions suchas scroll click tap key wheel etc until the long task hasfinished executing This is because when a long task isexecuting all user actions are queued behind the long taskPoorly designed JavaScript code is one example of what

might cause a browser main thread to block for over50ms [46] Since the current LongTask API (v1) does notreveal the URL of the long task (though the second versionof the API will provide such detail including the linenumber that caused the long task [45]) it is unclear as towhat particular resources block the browser main threadTherefore we only focus on investigating whether or notwe observe a rise in the total LongTask time when deviceCPU clock speed reduces when battery charge levels dropbelow a certain threshold as opposed to discussing the root

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

of battery-saver mode changes as network performanceimproves or degradesFinally similarly to how we calculate PLT distributions

we also calculate the TTFP observed for loading differentwebsites on different smartphones Additionally we collectthe total LongTask time the time to interactive and theframe rate observed when loading websites under variousbattery charge levels Note that since the LongTask timeto interactive and frame rate metrics depend on the devicehardware as opposed to the network performance for thesemetrics we do not calculate 10th 25th 50th 75th and 90thpercentile values (because we represent percentile valuesas network speed) but instead plot the whole distributionsin appropriate figuresNote that the dataset we prepared consists of 25 unique

combinations (comprising of country ISP URL smartphonename and HTTP protocol) for which mPulse library was exe-cuted on at least 100 page loads for each of the battery chargelevels ranging from 1 to 100 - a total of at least 10000 bea-cons for each combination Additionally the dataset consistsof 63 unique combinations for which mPulse library was ex-ecuted on at least 100 page-load transactions for at least 90battery charge levels - a total of at least 9000 beacons for eachcombination The low number of combinations observed inthe dataset is a consequence of the fact that only about 25of the total page loads occurred when battery charge lev-els were below 15 which yielded less than 100 beaconsfor some battery charge levels on some combinations Addi-tionally the low number of page loads under battery chargelevels less than 15 may suggest that either most users keeptheir phones charged over 15 at all times or that when bat-tery charge levels drop below 15 users reduce their Webbrowsing activities in favor of conserving battery charge

4 MEASURINGWEBSITE PERFORMANCETo estimate the performance of a website we analyze sev-eral Web performance metrics under different device batterycharge levels such as PLT and TTFP ndash the time since the startof the page navigation until the browser paints the first pixelsWe also analyze the total LongTask time ndash the time duringwhich the browser mainUI thread is blocked and thereforethe user cannot interact with the page [24] Finally we mea-sure the average rate of printing frames on the screen [27]

41 Measuring Page Load Time (PLT)411 Performance on Huawei Y6 Elite In Figure 1 we showvarious percentile PLT values for a page loaded on HuaweiY6 Elite smartphone From the figure we can observe thatacross all percentiles the PLT inflates as soon as the batterycharge level drops to 15 likely due to degraded CPU clockspeeds triggered by the battery-saver mode The figure also

0 20 40 60 80 100

6000

10000

14000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 1 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei Y6 Elite mobile device

Table 1 Summary of PLT distributions on Huawei Y6Elite phone

suggests that on most Huawei Y6 Elite smartphones thedefault threshold for when the battery-saver mode initiatesis set to 15In the Table 1 we summarize the analysis from Figure 1

to compare the PLT observed when battery charge levelsare for example 8 and 50 Note that the battery chargelevels 8 and 50 are just two example representative pointsfor comparing the performance under the battery-saver andnormal modes

From the table we can observe that the relative differencein the page load decreases as the network performancedegrades Specifically among the two battery levels 8 and50 for page loads represented by the 10th percentile distri-bution (p10) the PLTs at 8 are higher by 28 compared tothe PLTs at 50 Whereas for the 90th percentile distribution(p90) the PLTs at 8 are higher by 15 compared to thePLTs at 50 This downward trend suggests that whenwebsites are loaded on faster networks the smartphonersquosCPU becomes a more significant bottleneck for websiteperformance Indeed this trend is similar to a previousresearch study that investigates how device performanceimpacts Web performance [50]

0 20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75

Figure 2 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei P8 Lite (2015) mobile device

Figure 3 Summary of PLT distributions onHuawei P8 Lite phone

Both PLT and TTFP metrics make direct impact on theuser experience As such an increase in these metrics dueto degraded CPU clock speeds represents a degradation inthe user experience

412 Performance on Huawei P8 Lite (2015 model) Similarlyto Figure 1 as shown in Figure 2 we observe a suddeninflation in PLTs when loading a Web page on the 2015model of Huawei P8 Lite smartphone Since the inflationoccurs when the battery charge level is at 10 it appears thatthe battery-saver mode on the Huawei P8 Lite smartphoneturns on for most users at 10Additionally as shown in Table 3 when we compare the

relative PLT differences across different distributions wecan observe that the impact of throttled CPU clock speedson PLT appears to be more significant when pages areloaded in fast network conditions

We observed similar trends in performance for other web-sites that loaded on Huawei P8 Lite smartphone howeverwe do not show them due to page limits Additionally notethat for websites loaded on the 2017 model of Huawei P8 Litesmartphone we did not observe sudden inflation in PLT orTTFP metrics at any battery charge level potentially becausethe 2017 model has a faster CPU that does not impact thepage load despite the drop in CPU performance [8] Finallywe noticed that newer smartphones from the same family

20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 4 Page load time distributions across differentdevice battery charge levels as measured for a pageloaded on Sony Xperia Z5 Compact mobile device

such as Huawei P9 Lite and P10 Lite also do not degradewebsite performance under low battery charge levels

413 Performance on Sony Xperia Z5 Compact In Figure 4we show that when pages load on Sony Xperia Z5 Compactsmartphone there exists an upward slope indicating thatpage load times continue to rise as battery charge levelsdrop below 40 We have not been able to identify the causefor such an upward slope as opposed to a sudden increasedin page load time even though this device allows users toset a threshold at which the battery-saver mode activatesPerhaps this device has a linear slowdown in the CPU clockspeed when the battery charge levels reach 40 Note that wedid not observe such a strong upward slope for other Sonydevices such as Xperia X Compact Xperia X Performanceand Xperia XZ and in fact there was no inflation in PLT forthese devices under low battery charge levels

414 Performance on High-end Phones We compare thewebsite performance for pages loaded on various new andold Samsung flagship smartphones under different batterycharge levels Note that for the purposes of comparingperformance across different Samsung devices in Figure 5we only show the PLT values calculated for the 50thpercentile distribution at various battery charge levelsAdditionally note that for making the comparison easierto understand in Figure 5 we compare the performanceobserved for Huawei Y6 Elite smartphone for the samewebsite as the one used in Figure 1

From Figure 5 we observe that when the website isloaded on newer devices with high-performance CPU thePLT gets lower across all battery charge levels For examplethe PLTs on Note 8 are about 4 seconds whereas the PLTson Galaxy S6 are about 55 seconds Similarly PLTs onGalaxy S5 are about 65 seconds and the PLTs on Y6 Eliteunder non-throttled CPU hardware are about 8 secondsThe performance differences across devices show that the

0 20 40 60 80 100

4000

6000

8000

10000

Battery Level ()

PLT

(ms)

Note 8Galaxy S7

Galaxy S6Galaxy S5

Y6 Elite

Figure 5 PLT distributions across different device bat-tery charge levels as measured for a page loaded onvarious Samsung smartphones and Huawei Y6 Elite

0 20 40 60 80 100

1000

3000

5000

Battery Level ()

TTFP

(ms)

p10 p25 p50 p75 p90

Figure 6 TTFP distributions across different devicebattery charge levels as measured for a page loadedon Huawei Y6 Elite mobile device

website performance can be improved by upgrading thedevice hardware regardless of the battery charge levelAdditionally from Figure 5 we observe that PLTs on

Note 8 do not experience sudden or gradual increase at anybattery charge level Since Samsung Note 8 does not providea default user configurable option to enable the battery-savermode when the battery charge level reaches a certain thresh-old and that users must manually activate the battery-savermode perhaps most users tend to not activate the battery-saver mode and therefore we do not observe any inflation inPLTs Alternatively since Galaxy Note 8 has octa-core pro-cessors built-in perhaps even when the battery-saver modeis active the throttled CPU clock speed is high enough tonot negatively impact the page load process or the PLT [11]However we acknowledge that a better understanding of theinner workings of the device would help bear this out Forother devices such as Galaxy S7 and S6 we do not observeany inflation in PLT regardless of the battery charge levelSimilarly to Note 8 perhaps the throttled CPU clock speedson these devices are high enough to not impact the PLT

1000

2000

3000

4000

5000

Battery Level ()

Tota

l Lon

gTas

k Ti

me

(ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 7 LongTask time distributions across differentdevice battery charge levels as measured for a pageloaded on P8 Lite (2015) smartphone

42 Measuring TTFPIn Figure 6 we show various percentile TTFP values forthe same page as Figure 1 when loaded on Huawei Y6 Elitesmartphone From the figure we can observe that thereexists a sudden inflation in the TTFP when the batterycharge levels drop below 15 This trend suggests that thetime when the browser paints the first pixels also increaseswhen the device enables the battery-saver mode Similarlyto Figure 1 we also noticed sudden inflation in TTFP atbattery charge level 15 when loading websites on the 2015model of Huawei P8 Lite smartphone

43 Measuring LongTask TimeLongTask is a relatively new Web performance measure-ment API that allows identification of resources that makewebsites unresponsive to user interactions [19] Morespecifically Web developers could use the LongTask API todetect the presence of tasks that block the browser UImainthread for at least 50 milliseconds When a website loadsresources that block the browser UImain thread the useris unable to interact with the page Specifically a long taskprevents the page from responding to user actions suchas scroll click tap key wheel etc until the long task hasfinished executing This is because when a long task isexecuting all user actions are queued behind the long taskPoorly designed JavaScript code is one example of what

might cause a browser main thread to block for over50ms [46] Since the current LongTask API (v1) does notreveal the URL of the long task (though the second versionof the API will provide such detail including the linenumber that caused the long task [45]) it is unclear as towhat particular resources block the browser main threadTherefore we only focus on investigating whether or notwe observe a rise in the total LongTask time when deviceCPU clock speed reduces when battery charge levels dropbelow a certain threshold as opposed to discussing the root

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

0 20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75

Figure 2 PLT distributions across different device bat-tery charge levels as measured for a page loaded onHuawei P8 Lite (2015) mobile device

Figure 3 Summary of PLT distributions onHuawei P8 Lite phone

Both PLT and TTFP metrics make direct impact on theuser experience As such an increase in these metrics dueto degraded CPU clock speeds represents a degradation inthe user experience

412 Performance on Huawei P8 Lite (2015 model) Similarlyto Figure 1 as shown in Figure 2 we observe a suddeninflation in PLTs when loading a Web page on the 2015model of Huawei P8 Lite smartphone Since the inflationoccurs when the battery charge level is at 10 it appears thatthe battery-saver mode on the Huawei P8 Lite smartphoneturns on for most users at 10Additionally as shown in Table 3 when we compare the

relative PLT differences across different distributions wecan observe that the impact of throttled CPU clock speedson PLT appears to be more significant when pages areloaded in fast network conditions

We observed similar trends in performance for other web-sites that loaded on Huawei P8 Lite smartphone howeverwe do not show them due to page limits Additionally notethat for websites loaded on the 2017 model of Huawei P8 Litesmartphone we did not observe sudden inflation in PLT orTTFP metrics at any battery charge level potentially becausethe 2017 model has a faster CPU that does not impact thepage load despite the drop in CPU performance [8] Finallywe noticed that newer smartphones from the same family

20 40 60 80 100

4000

8000

12000

16000

Battery Level ()

PLT

(ms)

p10 p25 p50 p75 p90

Figure 4 Page load time distributions across differentdevice battery charge levels as measured for a pageloaded on Sony Xperia Z5 Compact mobile device

such as Huawei P9 Lite and P10 Lite also do not degradewebsite performance under low battery charge levels

413 Performance on Sony Xperia Z5 Compact In Figure 4we show that when pages load on Sony Xperia Z5 Compactsmartphone there exists an upward slope indicating thatpage load times continue to rise as battery charge levelsdrop below 40 We have not been able to identify the causefor such an upward slope as opposed to a sudden increasedin page load time even though this device allows users toset a threshold at which the battery-saver mode activatesPerhaps this device has a linear slowdown in the CPU clockspeed when the battery charge levels reach 40 Note that wedid not observe such a strong upward slope for other Sonydevices such as Xperia X Compact Xperia X Performanceand Xperia XZ and in fact there was no inflation in PLT forthese devices under low battery charge levels

414 Performance on High-end Phones We compare thewebsite performance for pages loaded on various new andold Samsung flagship smartphones under different batterycharge levels Note that for the purposes of comparingperformance across different Samsung devices in Figure 5we only show the PLT values calculated for the 50thpercentile distribution at various battery charge levelsAdditionally note that for making the comparison easierto understand in Figure 5 we compare the performanceobserved for Huawei Y6 Elite smartphone for the samewebsite as the one used in Figure 1

From Figure 5 we observe that when the website isloaded on newer devices with high-performance CPU thePLT gets lower across all battery charge levels For examplethe PLTs on Note 8 are about 4 seconds whereas the PLTson Galaxy S6 are about 55 seconds Similarly PLTs onGalaxy S5 are about 65 seconds and the PLTs on Y6 Eliteunder non-throttled CPU hardware are about 8 secondsThe performance differences across devices show that the

0 20 40 60 80 100

4000

6000

8000

10000

Battery Level ()

PLT

(ms)

Note 8Galaxy S7

Galaxy S6Galaxy S5

Y6 Elite

Figure 5 PLT distributions across different device bat-tery charge levels as measured for a page loaded onvarious Samsung smartphones and Huawei Y6 Elite

0 20 40 60 80 100

1000

3000

5000

Battery Level ()

TTFP

(ms)

p10 p25 p50 p75 p90

Figure 6 TTFP distributions across different devicebattery charge levels as measured for a page loadedon Huawei Y6 Elite mobile device

website performance can be improved by upgrading thedevice hardware regardless of the battery charge levelAdditionally from Figure 5 we observe that PLTs on

Note 8 do not experience sudden or gradual increase at anybattery charge level Since Samsung Note 8 does not providea default user configurable option to enable the battery-savermode when the battery charge level reaches a certain thresh-old and that users must manually activate the battery-savermode perhaps most users tend to not activate the battery-saver mode and therefore we do not observe any inflation inPLTs Alternatively since Galaxy Note 8 has octa-core pro-cessors built-in perhaps even when the battery-saver modeis active the throttled CPU clock speed is high enough tonot negatively impact the page load process or the PLT [11]However we acknowledge that a better understanding of theinner workings of the device would help bear this out Forother devices such as Galaxy S7 and S6 we do not observeany inflation in PLT regardless of the battery charge levelSimilarly to Note 8 perhaps the throttled CPU clock speedson these devices are high enough to not impact the PLT

1000

2000

3000

4000

5000

Battery Level ()

Tota

l Lon

gTas

k Ti

me

(ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 7 LongTask time distributions across differentdevice battery charge levels as measured for a pageloaded on P8 Lite (2015) smartphone

42 Measuring TTFPIn Figure 6 we show various percentile TTFP values forthe same page as Figure 1 when loaded on Huawei Y6 Elitesmartphone From the figure we can observe that thereexists a sudden inflation in the TTFP when the batterycharge levels drop below 15 This trend suggests that thetime when the browser paints the first pixels also increaseswhen the device enables the battery-saver mode Similarlyto Figure 1 we also noticed sudden inflation in TTFP atbattery charge level 15 when loading websites on the 2015model of Huawei P8 Lite smartphone

43 Measuring LongTask TimeLongTask is a relatively new Web performance measure-ment API that allows identification of resources that makewebsites unresponsive to user interactions [19] Morespecifically Web developers could use the LongTask API todetect the presence of tasks that block the browser UImainthread for at least 50 milliseconds When a website loadsresources that block the browser UImain thread the useris unable to interact with the page Specifically a long taskprevents the page from responding to user actions suchas scroll click tap key wheel etc until the long task hasfinished executing This is because when a long task isexecuting all user actions are queued behind the long taskPoorly designed JavaScript code is one example of what

might cause a browser main thread to block for over50ms [46] Since the current LongTask API (v1) does notreveal the URL of the long task (though the second versionof the API will provide such detail including the linenumber that caused the long task [45]) it is unclear as towhat particular resources block the browser main threadTherefore we only focus on investigating whether or notwe observe a rise in the total LongTask time when deviceCPU clock speed reduces when battery charge levels dropbelow a certain threshold as opposed to discussing the root

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

0 20 40 60 80 100

4000

6000

8000

10000

Battery Level ()

PLT

(ms)

Note 8Galaxy S7

Galaxy S6Galaxy S5

Y6 Elite

Figure 5 PLT distributions across different device bat-tery charge levels as measured for a page loaded onvarious Samsung smartphones and Huawei Y6 Elite

0 20 40 60 80 100

1000

3000

5000

Battery Level ()

TTFP

(ms)

p10 p25 p50 p75 p90

Figure 6 TTFP distributions across different devicebattery charge levels as measured for a page loadedon Huawei Y6 Elite mobile device

website performance can be improved by upgrading thedevice hardware regardless of the battery charge levelAdditionally from Figure 5 we observe that PLTs on

Note 8 do not experience sudden or gradual increase at anybattery charge level Since Samsung Note 8 does not providea default user configurable option to enable the battery-savermode when the battery charge level reaches a certain thresh-old and that users must manually activate the battery-savermode perhaps most users tend to not activate the battery-saver mode and therefore we do not observe any inflation inPLTs Alternatively since Galaxy Note 8 has octa-core pro-cessors built-in perhaps even when the battery-saver modeis active the throttled CPU clock speed is high enough tonot negatively impact the page load process or the PLT [11]However we acknowledge that a better understanding of theinner workings of the device would help bear this out Forother devices such as Galaxy S7 and S6 we do not observeany inflation in PLT regardless of the battery charge levelSimilarly to Note 8 perhaps the throttled CPU clock speedson these devices are high enough to not impact the PLT

1000

2000

3000

4000

5000

Battery Level ()

Tota

l Lon

gTas

k Ti

me

(ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 7 LongTask time distributions across differentdevice battery charge levels as measured for a pageloaded on P8 Lite (2015) smartphone

42 Measuring TTFPIn Figure 6 we show various percentile TTFP values forthe same page as Figure 1 when loaded on Huawei Y6 Elitesmartphone From the figure we can observe that thereexists a sudden inflation in the TTFP when the batterycharge levels drop below 15 This trend suggests that thetime when the browser paints the first pixels also increaseswhen the device enables the battery-saver mode Similarlyto Figure 1 we also noticed sudden inflation in TTFP atbattery charge level 15 when loading websites on the 2015model of Huawei P8 Lite smartphone

43 Measuring LongTask TimeLongTask is a relatively new Web performance measure-ment API that allows identification of resources that makewebsites unresponsive to user interactions [19] Morespecifically Web developers could use the LongTask API todetect the presence of tasks that block the browser UImainthread for at least 50 milliseconds When a website loadsresources that block the browser UImain thread the useris unable to interact with the page Specifically a long taskprevents the page from responding to user actions suchas scroll click tap key wheel etc until the long task hasfinished executing This is because when a long task isexecuting all user actions are queued behind the long taskPoorly designed JavaScript code is one example of what

might cause a browser main thread to block for over50ms [46] Since the current LongTask API (v1) does notreveal the URL of the long task (though the second versionof the API will provide such detail including the linenumber that caused the long task [45]) it is unclear as towhat particular resources block the browser main threadTherefore we only focus on investigating whether or notwe observe a rise in the total LongTask time when deviceCPU clock speed reduces when battery charge levels dropbelow a certain threshold as opposed to discussing the root

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

4000

6000

8000

10000

14000

Battery Level ()

TTI (

ms)

0 10 20 30 40 50 60 70 80 90 100

Figure 8 TTI distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

cause of a large or small total LongTask time Also notethat since LongTask time time to interactive and the framerate metrics do not depend on network speed and insteaddepend on the device CPU performance we use boxplotdistributions in Figures 7-9 instead of plotting differentpercentile values as we did for previous graphsIn Figure 7 we show the boxplot distributions of the

total LongTask time across different battery charge levelswhen loading a website on the 2015 model of the HuaweiP8 Lite smartphone From the figure we can observe thatthe total LongTask time inflates when device battery chargelevel drops below 10 The rise in the total LongTask timeindicates that when the device CPU clock speed is throttledto minimize battery consumption LongTasks block themain thread for longer than usual time Note that we didnot observe inflation in the total LongTask time on the2017 model of Huawei P8 Lite smartphone likely due to thefact that faster processors on the device do not impact thetotal LongTask time when their speeds are throttled by thebattery-saver mode

44 Measuring Time to Interactive (TTI)Not only does a LongTask delay user interactions butevents callbacks (such as onLoad) are also delayed In manypages where many LongTask exists as the page loads thetime at which the user could first interact with the pagecould also get delayed Additionally note that even thoughLongTask is the prime cause for poor responsiveness to userinteractions other tasks such as image decoding heavyrasterization work or presence of many layers on the pagecan also cause poor responsiveness [39]To investigate whether there is any impact of battery-

saver mode on the time when the user could first interactwith the website we take a look at the time to interactive(TTI) metric The TTI metric is calculated based on whenthe page was visually ready for the user and when the page

510

1520

2530

Battery Level ()

Fram

e R

ate

(FP

S)

0 10 20 30 40 50 60 70 80 90 100

Figure 9 FPS distributions across different device bat-tery charge levels as measured for a page loaded onP8 Lite (2015) smartphone

was ready for interaction [6] Specifically the former iscalculated by calculating the maximum of time to first paintand time to domContentLoadedEventEnd event [21] Oncethe time to visually ready is calculated the first time periodof 500 milliseconds during which the browser UImainthread was idle marks the TTI for the pageIn Figure 8 we show the boxplot distributions for TTI

values observed for loading a website on Huawei P8 Litesmartphone under different battery charge levels From thefigure we can observe that the TTI values inflate as soon asthe battery charge levels drop below 10 The sudden rise inTTI values indicates that users on this smartphone and othersimilar smartphones likely experience janks when interact-ing with the website thus leading to poor user experience

45 Measuring Frame Rate per SecondThe new requestAnimationFrame API allows Web develop-ers to strategically schedule paint events on the screen withthe goal of achieving a high frame rate during the websiteload [26] Typically 60 frames per second (FPS) is consid-ered ideal for good user experience which means that thebrowser has exactly 166 ms to produce a frame Howeverif the browser needs to perform tasks that delay the framegeneration the FPS declines Note that a low frame rate candegrade the user experience because under low FPS the pagebecomes unresponsive to user interactions Therefore weinvestigate whether or not the battery-saver mode impactsthe frame rate observed across different page loads Usingthe mPulse Continuity plugin we gathered the frame rate ob-served under various battery charge levels [6] Note that weshow analysis of frame rates only for one of the devices forwhich we observe a sudden rise in both the PLT and TTFP

As shown in Figure 9 the FPS observed on the 2015 modelof Huawei P8 Lite drops as soon as the device battery chargelevels drop below 10 This indicates that for websitesloading on this device as the CPU performance degrades

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

the rate at which the browser paints on the screen alsodeclines - leading to a potentially poor user experience

5 RELATEDWORKSeveral tools and studies have relied on active measurementsto identifyWeb performance bottlenecks [1 7 23 51] Unlikepassive measurements via real user monitoring systems [913] active measurement practices inherit several limitationsbecause pages loaded in controlled environments do notrepresent the characteristics of how pages load in the realworld [41] Steiner et al used a real usermonitoring system tocompare the impact of CPU processors embedded in old andnew smartphones on the web performance ndash suggesting thatpage loads on new devices with fast CPUs are significantlyfaster than page loads on old devices with slow CPU [50]Like other studies [42] the authors observed that faster CPUson newer smartphones load webpages faster than those onold phones Another study reveals that about 35 of the PLTis spent performing CPU-intensive tasks on user devices [51]

6 DISCUSSION AND CONCLUSIONSlow mobile device hardware is a bottleneck to mobile Webperformance We perform a large-scale measurement studyto identify the impact of Androidrsquos battery-saver (whichthrottles the CPU clock speed) mode on mobile Web perfor-mance Our data suggests that under low-battery conditionssudden rises in page load time total LongTask time andtime to interactive metrics are observed on some devicesThe average frame rate on some smartphones also declinesleading to unresponsive and paint-blocked websitesThrough this paper we hope we motivate the need

for new website design goals that improve mobile Webexperience for slow mobile devices The Web performancecommunity has developed numerous best practices fordevelopers to deliver high-performance experiences to endusers [4 25 29 30 35 48 49]

DISCLOSUREThe positions strategies or opinions reflected in this articleare those of the authors and do not necessarily represent thepositions strategies or opinions of Akamai

REFERENCES[1] 2009 Gomez Last-Mile Testbed httpsgooglBtwSWY (Nov 2009)[2] 2015 Navigation Timing httpw3cgithubionavigation-timing

(Aug 2015)[3] 2016 Mobile Trends in Retail httpswwwkountcom

LiteratureRetrieveaspxID=232433 (Mar 2016)[4] 2018 Accelerated Mobile Pages httpswwwampprojectorglearn

overview (Apr 2018)[5] 2018 Battery Status API httpsdevelopermozillaorgen-USdocs

WebAPIBattery_Status_API (Apr 2018)

[6] 2018 BOOMRplugins Continuity httpsakamaigithubioboomerangBOOMRpluginsContinuityhtml (Apr 2018)

[7] 2018 Cathpoint Synthetic Monitoring httpwwwcatchpointcomsynthetic-monitoring (Apr 2018)

[8] 2018 Compare Specs httpswwwgsmarenacomcomparephp3idPhone1=7201ampidPhone2=8516 (Apr 2018)

[9] 2018 Dynatrace Real user monitoring (RUM) httpswwwdynatracecomcapabilitiesreal-user-monitoring (Apr 2018)

[10] 2018 Extend battery life - Samsung Galaxy J5 httpswwwhelpforsmartphonecompublicensamsunggalaxy-j5android-5-1guides14Extend-battery-life-Samsung-Galaxy-J5 (Apr2018)

[11] 2018 Galaxy Note 8 Specs httpswwwgsmarenacomsamsung_galaxy_note8-8505php (Apr 2018)

[12] 2018 How do I use Power saving mode on my SamsungGalaxy S5 httpswwwsamsungcomuksupportmobile-deviceshow-do-i-use-power-saving-mode-on-my-samsung-galaxy-s5 (Apr2018)

[13] 2018 How mPulse works httpslearnakamaicomen-uswebhelpmpulsempulse-helpGUID-EBEC9222-7876-46F9-81A8-2227CFA89851html (Apr2018)

[14] 2018 How To Boost Your Battery httpssupportsonymobilecomusxperiaz5compactdmbattery (Apr 2018)

[15] 2018 How to exten battery life on HUAWEI Y6 Elitehttpswwwhardresetinfodeviceshuaweihuawei-y6-elitefaqtips-trickshow-to-save-battery-life-on-android-phone (Apr2018)

[16] 2018 How to Turn Battery Saver Mode On and Off on Alcatel Pixi 4httpssupportbellcaMobilitySmartphones_and_mobile_internetAlcatel-Pixi-4how_to_turn_battery_saver_mode_on_and_off_on_my (Apr 2018)

[17] 2018 How to Turn Power Saving Mode On and Off on SamsungGalaxy Note8 httpssupportbellcaMobilitySmartphones_and_mobile_internetSamsung-Galaxy-Note-8how_to_turn_power_saving_mode_on_and_off_on_my (Apr 2018)

[18] 2018 LG G5 Using Power Saving Mode httpsvideotrontmtxcaentopiclg_g5using_power_saving_modehtmlstep=4 (Apr 2018)

[19] 2018 Long Tasks API 1 httpsw3cgithubiolongtasks (Aug 2018)[20] 2018 Monitor and Extend Battery Life on Galaxy S6 httpswww

samsungcomussupportanswerANS00066391 (Apr 2018)[21] 2018 PerformanceTimingdomContentLoadedEventEnd https

developermozillaorgen-USdocsWebAPIPerformanceTimingdomContentLoadedEventEnd (Apr 2018)

[22] 2018 Sony Xperia Z5 and Z5 Compact Tips and Tricks httpswwwyoutubecomwatchv=T1BZNsNJBo4ampfeature=youtubeampt=71 (Apr2018)

[23] 2018 Test a websitersquos performance httpswwwwebpagetestorg(Apr 2018)

[24] 2018 Threading and Tasks in Chrome httpschromiumgooglesourcecomchromiumsrc+lkgrdocsthreading_and_tasksmd (Apr 2018)

[25] 2018 What Is Amazon Silk httpsdocsawsamazoncomsilklatestdeveloperguideintroductionhtml (Sept 2018)

[26] 2018 windowrequestAnimationFrame() httpsdevelopermozillaorgen-USdocsWebAPIwindowrequestAnimationFrame (Apr2018)

[27] K Basques 2018 Analyze Frames Per Second httpsdevelopersgooglecomwebtoolschrome-devtoolsevaluate-performanceanalyze_frames_per_second (Apr 2018)

[28] M Belshe R Peon and Ed M Thomson 2015 Hypertext TransferProtocol Version 2 (HTTP2) httpstoolsietforghtmlrfc6146 (May2015)

[29] Tiffany Brown 2012 Opera Mini and JavaScript httpsdevoperacomarticlesopera-mini-and-javascript (Sept 2012)

[30] Liu Chi 2018 How the Puffin Browser Works httpsmediumcomcoding-neutrino-bloghow-the-puffin-browser-works-440c91cece8f(Sept 2018)

[31] R Devine 2014 How to use battery saver on the LG G3 httpswwwandroidcentralcomhow-use-battery-saver-lg-g3 (Jul 2014)

[32] Tammy Everts 2013 The average web page has almost doubled insize since 2010 httpwwwwebperformancetodaycom20130605web-page-growth-2010-2013 (Jun 2013)

[33] Tammy Everts 2017 The average web page is 3MB How much shouldwe care httpsspeedcurvecomblogweb-performance-page-bloat(Aug 2017)

[34] Utkarsh Goel Moritz Steiner Mike PWittie Martin Flack and StephenLudin 2016 A Case for Faster Mobile Web in Cellular IPv6 NetworksIn ACM MobiCom

[35] Ilya Grigorik 2013 High Performance Browser Networking OrsquoReillyMedia Inc

[36] I Grigorik 2016 Fast and resilient web apps Tools and techniques -Google IO 2016 httpswwwyoutubecomwatchv=aqvz5Oqs238ampfeature=youtubeampt=19m45s (May 2016)

[37] I Grigorik 2018 Render-tree Construction Layout andPaint httpsdevelopersgooglecomwebfundamentalsperformancecritical-rendering-pathrender-tree-construction (Apr 2018)

[38] J Iyengar andM Thomson 2018 QUIC A UDP-BasedMultiplexed andSecure Transport httpswwwietforgiddraft-ietf-quic-transport-14txt (Aug 2018)

[39] Martina K 2016 Software vs GPU rasterization inChromium httpssoftwareintelcomen-usarticlessoftware-vs-gpu-rasterization-in-chromium (Feb 2016)

[40] MaxMind 2018 Detect Online Fraud and Locate Online Visitorshttpswwwmaxmindcomenhome (Apr 2018)

[41] P Meenan 2013 How Fast is Your Web Site In ACM Queue Volume11 issue 2

[42] Ashkan Nikravesh Hongyi Yao Shichang Xu David Choffnes andZ Morley Mao 2015 Mobilyzer An Open Platform for ControllableMobile Network Measurements In ACM MobiSys

[43] Erik Nygren 2015 Three years since World IPv6 Launchstrong IPv6 growth continues httpsblogsakamaicom201506three-years-since-world-ipv6-launch-strong-ipv6-growth-continueshtml (Jun 2015)

[44] Erik Nygren Ramesh K Sitaraman and Jennifer Sun 2010 TheAkamaiNetwork A Platform for High-performance Internet ApplicationsSIGOPS Oper Syst Rev 44 3 (Aug 2010)

[45] S Panicker 2016 Long Tasks APIv2 httpsdocsgooglecomdocumentd125d69JAC7nyx-Ob0a9Z31d1uHUGu4myYQ3os9EnGfdUedit(Sept 2016)

[46] S Panicker and N Jansma 2017 Reliably Measuring Responsivenessin the Wild httpswwwyoutubecomwatchv=y5qPix1tdOE (Jul2017)

[47] S Rutherford 2015 How to Turn on Battery Saver Mode in Android50 httpswwwtomsguidecomusbattery-saver-mode-android-5news-20491html (Mar 2015)

[48] S Souders 2008 High Performance Web Sites Essential Knowledge forFront-End Engineers OrsquoReilly Media Inc

[49] S Souders 2009 Even Faster Web Sites Performance Best Practices forWeb Developers OrsquoReilly Media Inc

[50] Moritz Steiner and Ruomei Gao 2016 What slows you down Yournetwork or your device In arXiv160302293

[51] Xiao Sophia Wang Aruna Balasubramanian Arvind Krishnamurthyand David Wetherall 2013 Demystifying Page Load Performance with

WProf In USENIX NSDI