So if this a runtime system issue, what about elisp on LLVM? Let the LLVM project handle the backend and performance issues, and emacs can maintain one language frontend. There are plenty of people working on that, so emacs can ride that for almost free.

I'm assuming that there are reasons why it doesn't work, as someone (apparently) did the work some time ago: https://github.com/boostpro/emacs-llvm-jit Perhaps it just needs a little TLC?