If your lsp-client code is doing anything where you need binding with a native library or the speed of emacs lisp is a factor, you are doing it wrong. All an lsp-client needs to do is make requests to the lsp-server, listen to the responses and update the UI. It literally doesn't matter how slow emacs-lisp is.

The only way an lsp-client could be slow is by making a request to the server and then blocking on the response - which is wrong and the package writer's fault because they are not using the asynchronous api emacs provides for communicating with a process.