Performance Audit Skill

Degree of freedom: MIXED — what to optimize [HIGH freedom]; measure-first Sentry/vitals and EXPLAIN [LOW freedom — run exactly].

Audit-and-fix exception. Measure, then optimize. JS payload → audit-bundle-size. Instant navigations → enhance-web-instant-nav. Breaking point → test-load. Timeouts/retries → audit-resilience. When fixing: change only the measured bottleneck and re-measure it; log other slow paths you notice as follow-ups, not fixes. Edit the existing file in place rather than rewriting it. Add a perf test only where the repo already keeps them or the task asks; scratch measurements stay out of the repo.

How to reason

1. Observe — quote LCP/INP/CLS, p95, or EXPLAIN / N+1 loop (file:line)
2. Interpret — is the user waiting on bytes, a query, or main-thread work?
3. Classify — vital-miss / N+1 / missing-index / bundle / image / re-render
4. Severity — poor LCP/INP or p95 >> target on a live path = Critical

Worked example