We report the first work that examines the internals of web browsers on smartphones, using the WebKit codebase, two generations of Android smartphones, and webpages visited by 25 smart-phone users over three months. We make many surprising findings. First, over half of the webpages visited by smartphone users are not optimized for mobile devices. This highlights the importance of client-based optimization and the limitation of prior work that only studies mobile webpages. Second, while prior work suggests that several compute-intensive operations should be the focus of optimization, our measurement and analysis show that their improvement will only lead to marginal performance gain with existing webpages. Furthermore, we find that resource loading, ignored by all except one prior work, contributes most to the browser delay. While our results agree with a recent network study showing that network round-trip time is a major problem, we further demonstrate how the internals of the browser and operating system contribute to the browser delay and therefore reveal new opportunities for optimization.