I have been a big fan of Firebug and YSlow for diagnosing Web site slowdowns, but they only work with Firefox. A free new tool, dynaTrace AJAX Edition, provides even more detail for Internet Explorer.
I downloaded and tested the tool today, and liked what I saw -- especially for free. As you can see in the image below (click on it to see it full size), dynaTrace AJAX Edition gives you a quick high-level view of where the time is being spent to load a page, including XMLHttpRequest calls and timer callbacks. It's not obvious from the picture, but it becomes clear when you use the tool that digging deeper is also fairly easy.
Giving away dynaTrace AJAX Edition for free is not entirely charity: It also gives dynaTrace the chance to plug its commercial editions to deliver an end-to-end, browser-to-server transaction-centric APM system.