If we're entertaining writing elisp methods in a faster language to improve overall performance, then it's already been done and being done via writing methods in C.
I don't see the point of polluting Emacs with Javascript.
Instructing users who want high-performance code in Emacs to write their code in C and rebuild their Emacs binary, or figure out how to make it a dynamic module, is about the most user-hostile way I can imagine to improve Emacs performance.
I don't see the point of polluting Emacs with Javascript.