I am sure people could find code such that an LLM can optimize it 10x fold. I have seen many types of code in my life (well written, horribly written, fast, slow, etc.).

I do feel there is a limit to LLM-s today. I did not try, but I doubt it will work if I would tell it "make me a web browser that is bug free, perfectly secure, works as efficient as possible on my architecture and has best UX for me personally".

Knowing where that limit is, is as hard as it always was knowing how fast a team of engineers was going to do a project.