Interesting read. When it comes down to it the answer seems to be that JavaScript needs to have support added for explicit parallelism. But isn't this what we're supposed to get with HTML5 web workers? It shouldn't be hard to write a utility library to implement parallel map, reduce, and so on in terms of spawning workers, falling back to serial execution if the worker API isn't available.
In either case, this will require a rewrite of every piece of code that needs it.
Speeding up/backgrounding the JIT compilation phase is a transparent optimization that I'm sure some smart people at Apple, Google, and Mozilla are already working on.