In this paper, we present Doloto, an optimization tool for Web 2.0 applications. Doloto analyzes application workloads and automatically rewrites the existing application code to introduce dynamic code loading. After being processed by Doloto, an application will initially transfer only the portion of code necessary for application initialization. The rest of the application’s code is replaced by short stubs—their actual implementations are transfered lazily in the background or, at the latest, on-demand on first execution of a particular application feature. Moreover, code that is rarely executed is rarely downloaded to the user browser. Because Doloto significantly speeds up the application startup and since subsequent code download is interleaved with application execution, applications rewritten with Doloto appear much more responsive to the end-user.