This is a good review of what looks like a decent study of the impact of AI on productivity. In this study, devs thought it sped them up, but in actuality they were slowed down. The article goes in depth into various possible explanations. The "this is one study" warning applies again - don't go around quoting this result as gospel truth, but it's fascinating nonetheless!
https://secondthoughts.ai/p/ai-coding-slowdown
https://metr.org/blog/2025-07-10-early-2025-ai-experienced-os-dev-study/