I'm extremely impressed by the performance of the JIT. The application I'm working has a lot of dependencies on JS behaviors that are initialized on page load. I've spent a lot of time to minimize the the impact of this code on page load, but there can be a good 250-1250ms delay (depending on PC hardware, the configuration of the page, etc.) before every element on the page is completely usable. So while there is a delay, I've designed things so that it should be pretty transparent to the user because they see the page immediately and by the time they'd actually go to do anything on the page, everything should be initialized.
While just testing the page under the latest nightly Firefox 3.1 build, this page is blazingly fast. It's so fast, I generally can't even see the initialization occur. I'm very impressed and I think things will only get better.
I definitely recommend reading John Resig's blog post on TraceMonkey. It contains a lot of technical detail as well as a brief overview on how it all works.
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