  Web browsers have been used widely by users to conduct various online
activities, such as information seeking or online shopping. To improve user
experience and extend the functionality of browsers, practitioners provide
mechanisms to allow users to install third-party-provided plugins (i.e.,
extensions) on their browsers. However, little is known about the performance
implications caused by such extensions. In this paper, we conduct an empirical
study to understand the impact of extensions on the user-perceived performance
(i.e., energy consumption and page load time) of Google Chrome, the most
popular browser. We study a total of 72 representative extensions from 11
categories (e.g., Developer Tools and Sports). We observe that browser
performance can be negatively impacted by the use of extensions, even when the
extensions are used in unintended circumstances (e.g., when logging into an
extension is not granted but required, or when an extension is not used for
designated websites). We also identify a set of factors that significantly
influence the performance impact of extensions, such as code complexity and
privacy practices (i.e., collection of user data) adopted by the extensions.
Based on our empirical observations, we provide recommendations for developers
and users to mitigate the performance impact of browser extensions, such as
conducting performance testing and optimization for unintended usage scenarios
of extensions, or adhering to proper usage practices of extensions (e.g.,
logging into an extension when required).
