I think the OP was suggesting that another language could be used for this purpose, similar in function to CoffeeScript in that it compiles down to JavaScript, but statically typed (or even just straight C). That way most of your app could be written in CoffeeScript or JavaScript, but for those few functions who's performance REALLY matters, you could reach for a different tool that makes writing this inhumane but performant flavor of JavaScript easy.

It's an interesting idea, and having little to no knowledge of compilers, I'd be interested in your take on it. Could emscripten already be used for something like this?