Interesting read. I wrote the original compiler back in 2002/2003, but a lot changed by the time it was open sourced (including the confusing name -- I just called it a javascript compiler).
One detail this story gets wrong though is the claim that, "The Gmail team found that runtime JavaScript performance was almost irrelevant compared to download times." Runtime performance was actually way more important than download size and we put a lot of effort into making the JS fast (keep in mind that IE6 was the _best_ browser at the time). One of the key functions of the js compiler was inlining and dead-code removal so that we could keep the code readable without introducing any extra overhead.
One detail this story gets wrong though is the claim that, "The Gmail team found that runtime JavaScript performance was almost irrelevant compared to download times." Runtime performance was actually way more important than download size and we put a lot of effort into making the JS fast (keep in mind that IE6 was the _best_ browser at the time). One of the key functions of the js compiler was inlining and dead-code removal so that we could keep the code readable without introducing any extra overhead.