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.
- Chrome Developer Tools - Chrome Developer Tools is a Chrome browser plugin that allows inspection of the DOM tree, HTML, CSS, request timeline, JavaScript execution, and has many other fine features.
- 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.
- Speed Tracer - Speed Tracer is a Chrome browser plugin that analyzes JavaScript execution and AJAX operations.
- Fiddler - Fiddler is an HTTP sniffer that provides detailed information about every request and response that goes through the network.