Keep in mind that a lot of performance differences come out of the snowball effect of popularity driving product development. X86 being faster than MIPS today has nothing to do with the relative merits of the ISAs and everything to do with the millions of people who wanted to run X86 code really fast for the past 25 years. JavaScript's foray into this scale of popularity is pretty recent, with the widespread adoption of "Web 2.0" and AJAX only dating back to about 2005 (both are older, but spent a while in the throes of obscurity and incompatibility).