  A Web browser utilizes a device's CPU to parse HTML, build a Document Object
Model, a Cascading Style Sheets Object Model, and render trees, and parse,
compile, and execute computationally-heavy JavaScript. A powerful CPU is
required to perform these tasks as quickly as possible and provide the user
with a great experience. However, increased CPU performance comes with
increased power consumption and reduced battery life on mobile devices. As an
option to extend battery life, Android offers a battery-saver mode that when
activated, turns off the power-hungry and faster processor cores and turns on
the battery-conserving and slower processor cores on the device. The transition
from using faster processor cores to using slower processor cores throttles the
CPU clock speed on the device, and therefore impacts the webpage load process.
We utilize a large-scale data-set collected by a real user monitoring system of
a major content delivery network to investigate the impact of Android's
battery-saver mode on various mobile Web performance metrics. Our analysis
suggests that users of select smartphones of Huawei and Sony experience a
sudden or gradual degradation in Web performance when battery-saver mode is
active. Battery-saver mode on newer flagship smartphones, however, does not
impact the mobile Web performance. Finally, we encourage for new website design
goals that treat slow (and throttled-CPU) devices kindly in favor of improving
end-user experience and suggest that Web performance measurements should be
aware of user device battery charge levels to correctly associate Web
performance.
