Optimize performance through mandatory measure-first workflow with 3 stages: MEASURE (gate) -> FUNDAMENTAL FIX -> CRITICAL PATH REDESIGN. Use when code is too slow, laggy, hangs, doesn't scale, has timeouts, OOM errors, or high CPU/memory. Emphasize that simpler code usually runs faster. Triggers on: performance issues, optimization needed, takes forever, profiling. Produces profiling-backed optimization with verified improvement.