A 2023 Microsoft study found that developers using GitHub Copilot completed coding tasks 55.8% faster than the control group, a figure that became an industry benchmark. But two years later, METR conducted a controlled experiment with experienced developers working on their own large codebases, yielding opposite results: AI tool users spent 19% more time, yet subjectively believed efficiency improved by 20%—a 39-percentage-point gap between subjective and objective measures.
Economists predicted a 39% acceleration, machine learning experts predicted 38%, both contradicting experimental results. This gap follows a clear pattern: AI feels efficient in the first 80% of tasks (basic framework generation, compilation, automated test generation), but the final 20% (system integration, edge cases, debugging legacy environments, understanding historical design trade-offs) is where real time is spent. Early speed gains mask later costs, leaving developers with no room to adjust when they realize the problem.