It seems to me that, given the incredible number of person-years put into making Javascript fast, it's probably approaching its max speed of execution already without modifying the language itself. I'd expect the C/C++ interpreters are already using so many tricks to speed it up that remaining gains would be difficult without adding more information to the language itself (e.g. stricter typing) or modifying the way it works (fewer things represented as objects, more types of collections).
In short, I suspect the actual instructions executed when JS runs on a modern interpreter are already pretty damn close to what a fairly well-optimized JS-to-WASM compiler would provide. If you want more major gains I think you'd have to start looking at locking down (or at least providing the option to lock down, in performance-critical code) the ability of Javascript to do all kinds of wacky stuff at runtime.
In short, I suspect the actual instructions executed when JS runs on a modern interpreter are already pretty damn close to what a fairly well-optimized JS-to-WASM compiler would provide. If you want more major gains I think you'd have to start looking at locking down (or at least providing the option to lock down, in performance-critical code) the ability of Javascript to do all kinds of wacky stuff at runtime.