Experienced developers were 19% slower with AI tools, study finds
A 2025 randomized controlled trial by METR found that experienced open-source developers completed coding tasks 19% slower when using AI assistance compared to working without it. The study involved seasoned developers working on their own familiar repositories, making the slowdown particularly notable on codebases they knew best. Before and after each task, participants consistently believed AI had made them faster — estimating gains of around 20-24% — revealing a significant gap between perception and measured reality. Researchers attributed the slowdown to the overhead of prompting, reviewing, and correcting AI output, which added time on top of work developers could already perform quickly from memory. The study also noted that developers who avoided the productivity loss were those who treated AI tool effectiveness as a measurable question rather than an assumption, tracking their own performance rather than relying on intuition.
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