llms remove a lot of the difficulty of writing a ton of reasonable code, but is that really the bottleneck to producing a bunch of PRs?

isn't it the reviewing time? reviewing code is hard work

Reviewing code can be hard but it's not as hard as writing the code. Even with the best autocomplete, and ergonomic editors like vim, it still takes quite a bit of time to write code for some features compared to the actual concepts being implemented. There are also lots of decisions like variable names that can be automated with a LLM. If you don't like what it came up with, you can tell it to change them. I recommend that you keep them fairly unique like you would for your own handwritten code, because ambiguity creates problems for people and machines alike.