Boomerang is totally awesome! Really useful. Thanks for sharing.
I've noticed a very small but important bug. The unload event in the RT plugin should be attaching itself to the "beforeunload" event rather than "unload" because by the time unload fires, the next page has already started loading on the server so it under-estimates the page load time. This is hardly noticeable with normal HTML pages but when you have a dynamic page where the server has to do work to generate the page it's much more noticeable.
There are two reasons why we don't use the onbeforeunload event.
1. It doesn't work in Opera. Opera never has, and doesn't plan to support the onbeforeunload event, which means we can't use it. We could potentially use it on all browsers except opera, falling back to onunload for Opera, but...
2. In IE, any page that has an onbeforeunload handler will not be cached in the browser. I haven't tested if this is still true with newer versions of IE, or if it is a problem with other browsers, but I don't think a tool that measures performance should be the reason performance reduces, and preventing caching would definitely reduce page performance.
Update: We now use the onbeforeunload event, and fallback to onunload where it isn't supported