A skill / prompt for any Rails app where the test suite is too slow. Took one app from 120s → 36s wall-clock by walking this list. Use it as an investigation playbook, not a checklist — every project has a different worst offender, but the same handful of patterns produce most of the slowness.

The order matters: profile first, fix code-side, then attack infra. Throwing CPU at a slow suite buys you a 30 % discount on something you should have crushed at the source.