Some people are saying that https://github.com/ubolonton/emacs-tree-sitter will eventually replace the default regexp-based font-lock for syntax highlighting. I'm personally testing it out and it's been great!
Apparently, Emacs' display code shares the blame, and no one wants to touch it as it's written in some really hairy legacy C.
Though I'm not a Rust fan, and would infinitely prefer Guile Emacs, I am kind of half-hoping Remacs succeeds so Emacs can finally put all of that legacy C behind it.
Anyway, while elisp might not be the fastest language around, it's not the origin of all slowness in emacs. Crufted design is also a big culprit. The dated regex-machine is often mentioned as a big problem, making syntax highlighting especially painful.