Here is list of my personnel how to, it's mostly for me to help me remember how I at some point figure out how to do something...
PS: if you like to contribute to my How To blog, let me know and I will give you access to do so.
Everything on the internet change so fast, I'm sure the minute I will post this other better tools may show up, but for 9/2011, those tools may help you out finding performance issues on your AJAX web application.
Firebug - Firebug is a Firefox browser plugin that provides much of the same functionality as the Chrome Developer Tools.
FireCookie - FireCookie is a plugin to Firebug that allows detailed inspection of browser cookies.
Page Speed - Page Speed is a plugin for Chrome and Firefox browsers that provides detailed analysis and recommendations around Web 1.0 best practices.
YSlow - YSlow is a plugin for Chrome and Firefox browsers that provides detailed analysis and recommendations around Web 1.0 best practices. In spite of significant overlap, we tested with Page Speed and YSlow, because they provide different perspectives and, in certain cases, different opinions.
Fiddler - Fiddler is an HTTP sniffer that provides detailed information about every request and response that goes through the network.