https://bugzilla.mozilla.org/show_bug.cgi?id=840282 has some measurements on how fast the implementation of asm.js (called OdinMonkey) already is.
"sm" is SpiderMonkey, that is the normal JS engine. v8 is Chrome's JavaScript engine.
Current results on large Emscripten codebases are as follows, reported as factor slowdown compared to gcc -O2 (so 1.0 would mean "same speed")
I kinda hope that someone at Mozilla keeps a count of just how many monkeys have gone into the code base. Off the top of my head, there's SpiderMonkey, TraceMonkey, JagerMonkey, IonMonkey, ScreamingMonkey, IronMonkey, and perhaps you should count Tamarin. It would be neat to see mascot like versions of them all... :)
IronMonkey was apparently for "mapping IronPython and IronRuby (and maybe IronPHP) to Tamarin". I just vaguely recalled seeing the name pop up in a Mozilla related context -- I certainly couldn't have told you what it was without looking it up. :)
Current results on large Emscripten codebases are as follows, reported as factor slowdown compared to gcc -O2 (so 1.0 would mean "same speed")