A recent study has thrown a wrench in the widely held belief that AI tools significantly accelerate software development. Researchers at METR conducted a randomized controlled trial with experienced developers using AI-enhanced tools like Claude 3.5 and Cursor Pro. Surprisingly, they found that developers using AI assistance took 19% longer to complete tasks compared to those who worked without it, despite their belief that they were being more efficient. The study involved 16 professional developers tackling real-world tasks in mature open-source projects, revealing a stark contrast between perceived productivity and actual outcomes.