As the complexity of your Ajax applications increase, so do the opportunities for performance issues to arise. Checkout TIBCO General Interface Performance Profiler, an open source (BSD License) Ajax application profiling tool for benchmarking the performance of Ajax solutions created with TIBCO General Interface and other Ajax toolkits. Create performance testing scripts in JavaScript then run them against live applications to get performance statistics for selected operations within your application. The Performance Profiler is smart enough to let you handle asynchronous operations too.
With this profiler, you can now dissect your application to test how long it takes your services to respond, your components to render, your data to parse, and your functions to execute. Basically everything you need to pinpoint the bottlenecks in your code. One thing this profiler won't do -- optimize your code for you -- that's your job.