Our team, in conjunction with Microsoft Research, have been working on a power tool known as AjaxView which helps measure the performance of web applications that execute JavaScript inside the web browser.  This power tool provides an IIS extension that automatically adds profiling code into a JavaScirpt-based web application as well as a Visual Studio add-in to view the profiling data in the Visual Studio Performance Explorer. 

Check out the details on Soma’s blog: VS2008 AJAX Profiling Extensions