This paper conducted a randomized controlled trial (RCT) between February and June 2025 to understand the impact of AI tools on the productivity of experienced open source developers. Sixteen developers (with a medium level of AI experience) performed 246 tasks on projects with an average of 5 years of experience. Each task was randomly assigned whether or not to allow the use of AI tools, and when allowed, they primarily used Cursor Pro and Claude 3.5/3.7 Sonnet. Developers expected that allowing the use of AI tools would reduce task completion time by 24%, but in reality, it increased it by 19%. This result is also contrary to the predictions of economists (who expected a 39% reduction) and ML experts (who expected a 38% reduction). The study analyzed 20 characteristics that could affect the slowdown effect (e.g., project size, quality criteria, and developers’ experience with AI tools) and concluded that although the influence of experimental artifacts cannot be completely ruled out, the robustness of the slowdown effect across the analysis makes it difficult to explain it solely as a problem of the experimental design.