I often felt that current ML speeds up newbie devs by effectively teaching them the language and libraries — but slows down experts that already know the stack well from memory.

It depends. You don't need LLMs to write stuff for you that you already know. You use them to take.care of the drudge work or explore things you are not familiar with. Things like Copilot's /explain can speed up onboarding even for seasoned developers, and Copilot can also help you speed up iterations on proofs of concept. For example, I've been using Copilot to experiment with some changes to the software architecture of some projects I own, and it's fantastic at generating code following specific design patterns. It's also fantastic to get it to iterate designs in near-real.time by prompting it with directives such as "repeat the last example but implementing X with design pattern Y and moving the implementation to Z". You are presented with examples that you can browse through and get a taste of what you'd get, but with a fraction of the time. To top things off, you can prompt Copilot to present pros and cons and even propose optimizations.

Like any tool, it has its purposes. You just need to learn how to use it.