https://metr.org/blog/2025-07-10-early-2025-ai-experienced-os-dev-study/

Core Result

When developers are allowed to use AI tools, they take 19% longer to complete issues—a significant slowdown that goes against developer beliefs and expert forecasts. This gap between perception and reality is striking: developers expected AI to speed them up by 24%, and even after experiencing the slowdown, they still believed AI had sped them up by 20%.

Note, this test was run on tickets or tasks that developers anticipated taking about 2 hours a piece. So that wouldn't cover AI helping a front end developer trying to do something novel in the back end.