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?