> if you just let it rip on the problem for hours

In my experience, you can get good results if Fable does't write code itself, only spawn subagents.

I can run Fable for 10 hours, and it would output 50k tokens and read 300k (30% of the context window). The resulting code is okay-ish. I would rarely merge LLM-produced code first try without an adversary review.