I am excited by the performance measurement tools I see upon the horizon. For years many web developers focused on improving the performance of the web server, and the benefits of future optimization on the server are diminishing. Now the tools and techniques allowing us to systematically improve client side performance are arriving. One such tool is Episodes from Steve Souder. Out of the box, Episodes shows the breakdown of response time between back end and front end processing. But the real treasure in the product is the ability to break down many aspects of the client side processing time to see how each piece performs. This should help you identify what javascripts are taking a long time to process. You can even collect live performance data from actual users.
You can find the product on the web at www.SteveSouders.com. Steve Souders is the author of Yslow, another excellent Firebug for Firefox addin.
Rob Kraft's Software Development Blog 