Experienced devs felt 20% faster with AI, measured 19% slower in controlled trial

The gauge broke: devs felt 20% faster with AI, measured 19% slower

METR ran a controlled trial with 16 experienced open-source devs across 246 tasks in familiar codebases. Measured time was ~19% slower with frontier AI tools, while self-reported speed was ~20% faster—a nearly 40-point gap. Faros AI data across 10,000+ devs shows PRs merged up 98%, PR size up >150%, review time up 91%, with roughly no net delivery change and 31% of PRs merged without review. GitClear's analysis of 200M changed lines found copy-pasted code rising and refactoring falling below 10% of changes. The bottleneck shifted from writing to reviewing, but most dashboards still track the old metric. The slowdown flips positive for juniors and greenfield work; the author frames this as a likely J-curve dip, not the final state.

Why it matters: METR's controlled trial is the first to quantify the AI coding speed illusion with a stopwatch—16 experienced devs, familiar codebases, 20% perceived gain vs 19% measured loss. The sample is small and the full methodology isn't disclosed in the excerpt, but the counterintuitiv...