It wasn't until I read your comment that I was able to pinpoint why the mental exhaustion feels familiar. It's the same kind (though not degree) of exhaustion as formal methods / proofs.

Except without the reward of an intellectual high afterwards.

I feel this too. I suspect its a byproduct of all the context switching I find myself doing when I'm using an LLM to help write software. Within a 10 minute window, I'll read code, debug a problem, prompt, discuss the design, test something, do some design work myself and so on.

When I'm just programming, I spend a lot more time working through a single idea, or a single function. Its much less tiring.

In my experience it's because you switch from writing code to reviewing code someone else wrote. Which is massively more difficult than writing code yourself.