The main reason my emacs is painfully slow is that most elisp code blocks. Stuff like hitting tab for auto-completion might perform a blocking query from a language server hanging emacs for multiple seconds. Opening a file tries to start a language server and blocks the editor in the process. Etc.
Making emacs itself faster won't make the experience of working with emacs any better.
Note that multithreading is now in Emacs master, and some core packages (e.g., Tramp, which used to block when opening a remote file) have already been updated to make use of it, so starting with the next major release things will be improving on this front.
I agree that it's not the panacea though, Emacs would benefit from using a more threaded or event-oriented programming model for many things.
Though I only use auto-complete on emacs commands, so maybe I've just missed it.