Microsoft research recently released a new tool to help developers with Ajax applications. You can check out the site for more information, Ajax View.
This is some really cool technology that basically puts a proxy in between the server and the end-users browser and allows instrumentation and other really useful things to happen.
You can see a simple walkthrough Ajax View's performance profiling.
If you haven’t looked at the various things that our research team is working on, I would highly suggest you check it out. Things like:
This is just a sample of some of the really cool work then are doing. Check out the main page at http://research.microsoft.com