20 hours of work turns into 20 minutes
The gains, where they exist, are nowhere near that much. In some cases, it makes developers slower (even though they think they're a bit faster):
we find that when developers use AI tools, they take 19% longer than without - AI makes them slower.
https://metr.org/blog/2025-07-10-early-2025-ai-experienced-os-dev-study/