perf

# Perf — measure, find the real bottleneck, fix the biggest, verify Slowness has a **real, measurable cause**. The cardinal sin is optimizing by intuition — you'll spend effort on the wrong thing and maybe trade away correctness for nothing. Get data first. ## The loop 1. **Reproduce + measure.** Get a real number under a realistic scenario — wall-clock, FPS/frame time, query ms (`EXPLAIN ANALYZE`), request latency, bundle size, memory. No number, no optimizing. Write it down; it's your before. 2. **Find the bottleneck — profile, don't guess.** Use the right instrument (browser Performance panel / React Profiler, a flame graph, DB query plan, a tracer) and find *where the time actually goes* — the ~20% causing ~80%. The universal classes of waste: **repeated work** (recomputed per item/render instead of once), **amplified work** (one request fanning out into N), **missing lookup structure** (a scan where an index/map belongs), **serial waiting** (round-trips that could be batched or parallel), **oversized payloads**, and **no caching of stable results**. For the *domain-specific* suspects, load the active field pack (`engineering/active-field.md` → the pack's performance section); if the field has no pack, let the profile — not a checklist — name the suspect. 3. **Fix the biggest one.** Make the single change with the most impact; resist micro-optimizing noise. Prefer *doing less work* (cache, batch, index, memoize, defer, paginate) over doing...