Speed Tracer is a tool to help you identify and fix performance problems in your web applications. It visualizes metrics that are taken from low level instrumentation points inside of the browser and analyzes them as your application runs. Speed Tracer is available as a Chrome extension and works on all platforms where extensions are currently supported (Windows and Linux).
something similar, i’m still a big fan of fiddler: http://teusje.wordpress.com/2011/02/11/fiddler-a-web-debugging-proxy/
Fiddler only shows loading times and responses when making HTTP requests, such as Firebug and the other developer tools can do. Speed Tracer can actually show you how an app performs while you are using it.
One of the key features Speed Tracer has, is that it can tell you when the UI Thread is being blocked. Slicknet has a brilliant presentation on the UI Thread and slow(ish) Javascript which explains the whole deal
maybe it can be done via an extension (didn’t check them all) http://www.fiddler2.com/Fiddler2/extensions.asp