v8 is not C-fast on general code.

A lot of people seem to have the mistaken impression that v8 makes Javascript "fast". It's "fast" for a dynamic language. But on general code... it's still slow. It seems to plateau around 10x slower than C, as with the other JIT efforts to speed up dynamic languages, with a roughly 5-10x memory penalty in the process.

Microbenchmarks like the benchmark game tend to miss this because a lot of microbenchmarks focus on numeric speed. But numeric code is easy mode for a JIT. Now, that's cool, and there's nothing wrong with that. If it's the sort of code you have, great! You win. But that performance doesn't translate to general code. These are not value judgments, these are just facts about the implementation.

I expect v8 is roughly as fast as JS is going to get, and it's now news if they can eke out a .5% improvement on general code.

You can also do much better with v8 if you program in a highly restricted subset of JS that it happens to be able to JIT very well. However, this is not really the same as writing in JS. It's an undocumented subset, it's a constantly changing subset, and there's not a lot of compiler support for it (I'm not aware of anything like a "use JITableOnly" or anything).

The problem is not jitting pure Python code. PyPy does that and it's quite good at doing it. The problem is exactly what mitsuhiko says, and lots of Python apps use some or all of these features implicitly (through the native extensions used by many common dependencies). Sure, some of this madness can be accessed from Python itself, but that's also the case for JS, where you can do certain things, which will slow down your code greatly because the JIT can't work with it.