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performance

Use when a page or interaction is slow for one user and the fix starts from a measurement — a failing Core Web Vital (LCP, INP, CLS), a heavy JS bundle, wasted re-renders, an over-broad client boundary: profile, attribute the cost to one phase, fix it, re-measure. NOT surviving concurrent load (that is `scaling`), NOT the slow query (that is `postgresdb`).

skills/performance/SKILL.md

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# Performance: make one thing fast

Performance makes a single request, page, or interaction faster for one user. Scaling makes many requests survive at the same time. Different problem, different toolbox — if the system is fine for you and only bends under concurrent load, that is `../scaling/SKILL.md`, not this.

The whole job in one line: **measure with a profiler → attribute the cost to one specific phase → apply the one fix that moves that phase → re-measure against a number.** Repeat until you clear the threshold, then stop.

**Prime directive: no fix without a profile first.** A waterfall, a flame chart, a bundle treemap, or a field CWV number comes before you touch code. The fix you guess is almost never the fix the measurement points at, and a "fast" change that moves a phase nobody was waiting on is wasted work that you then have to maintain.

Cross-reference, don't duplicate: the last column of the table below names where each deep fix lives. The one case it has no row for — you decided *what* to cache to cut a phase and now need that cache race-free — is `../redis/SKILL.md`.

## Decision table — symptom to first move

| Symptom | Metric / phase | First probe | First lever | Deep fix lives in |
|---|---|---|---|---|