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

Observe: Sentry p95 /users = 2.3s; handler findAll then per-user Order.findByUserId (app/api/users/route.ts:31). Interpret: N+1; TTFB/LCP suffer on that page. Classify: N+1. Severity: Critical (p95 far over 200ms). Finding: /users | N+1 | eager-load orders | then re-measure

Step 0: Gather Performance Data [LOW freedom — run exactly]

Before optimizing, measure. Collect data from multiple sources.

Production Data (Sentry)

If Sentry performance monitoring is enabled, fetch real production metrics:

sentry:search_events
{
 "organizationSlug": "<ORG_SLUG>",
 "projectSlug": "<PROJECT_SLUG>",
 "regionUrl": "<REGION_URL>",
 "query": "slowest transactions by p95 duration in last 7 days",
 "limit": 20
}

sentry:search_events
{
 "organizationSlug": "<ORG_SLUG>",
 "projectSlug": "<PROJECT_SLUG>",
 "regionUrl": "<REGION_URL>",
 "query": "web vitals LCP INP CLS performance scores in last 7 days",
 "limit": 20
}

Check for performance-related issues:

sentry:search_issues
{
 "organizationSlug": "<ORG_SLUG>",
 "projectSlugOrId": "<PROJECT_SLUG>",
 "regionUrl": "<REGION_URL>",
 "query": "performance issues slow queries N+1 in last 30 days",
 "limit": 20
}

Research Current Benchmarks

Fetch current performance targets:

firecrawl:firecrawl_search
{
 "query": "web vitals thresholds good score <current year>",
 "limit": 3,
 "sources": [{ "type": "web" }]
}

Field Data (source of truth) [LOW freedom — run exactly]

Lab (Lighthouse) is for diagnosis; field decides pass/fail.

1. CrUX / PageSpeed Insights field tab for the origin and top 5 URLs by traffic — record p75 LCP / INP / CLS (mobile first).
2. web-vitals/attribution in the app (not the base build): it names the LCP element, the INP interaction target, and the CLS-shifting node.
3. Lighthouse CI with a budget.json (see references/loading-priority-2026.md §Budgets). Fail the PR on regression, not on absolute score.

Record all three in the report's "Production Metrics" block before touching code.

Verification protocol [LOW freedom — run exactly]

# Field (source of truth): CrUX / PSI field tab → p75 LCP, INP, CLS
# Lab, repeatable, budgeted:
npx @lhci/cli autorun --config=lighthouserc.json
# Early Hints present?
curl -sv https://example.com/ 2>&1 | grep -E "< HTTP/.* 103"
# Speculation: DevTools → Application → Speculative loads
# bfcache:     DevTools → Application → Back/forward cache → Test
# LCP element: web-vitals/attribution → onLCP(({attribution}) => attribution.element)

Performance Targets

Core Web Vitals

Other Key Metrics

• TTFB: <800ms field p75 (Google threshold); aim <200ms at the edge
• FCP: <1.8s
• Long tasks: none >50ms on the critical interaction path (INP driver)
• LCP resource load delay: <100ms (gap between TTFB and the LCP image request starting — usually "image not discoverable / not preloaded")
• bfcache: eligible on every content page (DevTools → Application → Back/forward cache)

Frontend Performance Audit [HIGH freedom]

Bundle Size Analysis

npm run build -- --analyze # framework-specific
npx source-map-explorer 'dist/**/*.js'

Checklist:

• Total bundle <200KB gzipped
• No single chunk >100KB gzipped
• Tree shaking working (no unused exports in bundle)
• Heavy libraries lazy-loaded (charts, editors, maps)
• No moment.js (use date-fns or dayjs)
• No lodash full import (use lodash-es or specific imports)

Code Splitting

• Route-based splitting (each page loads its own chunk)
• Component-level splitting for heavy components (lazy() / dynamic())
• Below-fold content deferred

Image Optimization

• WebP/AVIF format used (not PNG/JPEG for photos)
• Images sized appropriately (not serving 4K to mobile)
• loading="lazy" on below-fold images
• Responsive srcSet for different screen sizes
• Image CDN used (Cloudinary, imgix, Vercel Image Optimization)

Loading Priority & Speculation [HIGH freedom on which to apply, LOW freedom on the LCP rules]

Deep syntax: references/loading-priority-2026.md. Instant-nav implementation → enhance-web-instant-nav.

LCP element (find it first with web-vitals/attribution or DevTools → Performance → LCP)

• LCP image is not loading="lazy" — lazy on the LCP image is an automatic Critical finding
• LCP image has fetchpriority="high" (Next.js: <Image priority />, which also injects the preload)
• LCP image is discoverable from HTML — not a CSS background-image, not injected by client JS, not behind a hydration boundary
• <link rel="preload" as="image" imagesrcset imagesizes> when the LCP image is responsive and not already preloaded by the framework
• Non-critical images: loading="lazy" decoding="async"; <picture> AVIF → WebP → JPEG fallback

Fonts

• font-display: swap (or optional for non-brand text); fallback metrics set (size-adjust, ascent-override) so swap causes no CLS — next/font does this automatically, verify it's actually used
• Preload only the above-fold weight(s); subset to used unicode ranges; self-host

Early Hints (103)

• Origin/CDN sends 103 Early Hints with preconnect for the image/font CDN and preload for the LCP image + critical CSS. Verify: curl -sv https://site/ 2>&1 | grep -i "< HTTP/.* 103"
• Node: res.writeEarlyHints({ link: [...] }); otherwise configure at the CDN

Speculation Rules (next-navigation speed)

• <script type="speculationrules"> present with prefetch document rules at moderate eagerness for in-site links
• prerender only for high-probability next pages (pagination, "next article", product detail); eagerness moderate/conservative
• Excluded from speculation: logout, cart/checkout, auth, anything with side effects on GET, URLs with ? unless expects_no_vary_search is set
• Server ignores side-effects when Sec-Purpose: prefetch;prerender is present; analytics/ads guarded by document.prerendering + prerenderingchange
• Full implementation → enhance-web-instant-nav

bfcache

• No unload listeners (use pagehide); no Cache-Control: no-store on HTML; no open WebSocket / IndexedDB transaction at pagehide
• Verified green in DevTools → Application → Back/forward cache → "Test back/forward cache"

Render strategy (Next.js 16 / React 19)

• Server Components by default; 'use client' only at interactive leaves
• Slow data wrapped in <Suspense> so the shell streams; static shell + dynamic holes (cacheComponents / 'use cache') instead of blocking the whole route
• React Compiler enabled (reactCompiler: true) before any manual memo/useMemo/useCallback — manual memo only where Profiler proves the Compiler missed it

INP (responsiveness)

• No task >50ms on tap/click/keypress paths; break work with scheduler.yield() (fallback setTimeout(0)) or startTransition
• No synchronous layout reads (offsetHeight, getBoundingClientRect) inside input handlers
• content-visibility: auto + contain-intrinsic-size on long off-screen sections
• Third-party scripts: async/defer, Next <Script strategy="lazyOnload"> (or "worker" via Partytown where safe), consent-gated, facades for embeds (lite-youtube, static map image → iframe on click)

CLS

• Every <img>, <video>, <iframe>, ad slot has width/height or aspect-ratio
• Late-arriving UI (banners, toasts, cookie bars) reserves space or overlays — never pushes content

React Performance (if applicable)

• React Compiler on; manual memo only with Profiler evidence (see Loading Priority & Speculation)
• No unnecessary re-renders (React DevTools Profiler)
• Long lists virtualized (react-window, @tanstack/react-virtual)
• No inline object/array creation in JSX props
• Context providers scoped narrowly (not wrapping entire app for local state)

CSS Performance

• No layout thrashing (reads before writes)
• Animations use transform/opacity (GPU-accelerated)
• No @import chains (bundled instead)
• Critical CSS inlined for above-fold content
• Fonts subset and preloaded (<link rel="preload">)

Backend Performance Audit [HIGH freedom]

Database Queries

N+1 Detection:

// BAD: N+1
const users = await User.findAll();
for (const user of users) {
 const orders = await Order.findByUserId(user.id);
}

// GOOD: eager loading
const users = await User.findAll({ include: [Order] });

Checklist:

• N+1 queries eliminated (eager loading, joins, batch loading)
• Indexes on frequently queried columns (WHERE, JOIN, ORDER BY)
• SELECT only needed columns (no SELECT *)
• Pagination on all list endpoints
• Connection pooling configured
• Slow query logging enabled

Index verification: [LOW freedom — run exactly]

EXPLAIN ANALYZE SELECT * FROM orders WHERE user_id = '123';

API Response Optimization

• Response compression enabled (gzip/brotli)
• Appropriate cache headers (Cache-Control, ETag)
• No over-fetching (return only what the client needs)
• Pagination for list endpoints
• Response time <200ms for p95

Caching Strategy

Network Optimization

• CDN for static assets
• HTTP/2 or HTTP/3 enabled
• Preconnect to critical origins (<link rel="preconnect">)
• DNS prefetch for third-party domains
• API calls batched where possible (GraphQL, DataLoader)

Research-Driven Optimization [HIGH freedom]

For specific performance bottlenecks, research current solutions:

firecrawl:firecrawl_search
{
 "query": "<framework> <specific bottleneck> performance optimization <current year>",
 "limit": 5,
 "sources": [{ "type": "web" }]
}

Then deep-read the best result:

firecrawl:firecrawl_scrape
{
 "url": "<best-result-url>",
 "formats": ["markdown"],
 "onlyMainContent": true
}

Check official framework docs via Context7:

context7:resolve-library-id
{
 "libraryName": "<framework>",
 "query": "performance optimization"
}

Quick Wins (Highest Impact, Lowest Effort)

Self-critique before applying fixes [LOW freedom — do not skip]

1. Evidenced — a measured vital, p95, or EXPLAIN, not "this looks slow"
2. Reproducible — same path still exceeds the target table
3. Severity justified — Critical = poor vital or live-path p95
4. Right owner — JS payload → audit-bundle-size; breaking point → test-load; timeouts → audit-resilience
5. No-false-safety — no memo/useMemo without a measured re-render; measure again after the fix
6. LCP named — prove which element is LCP before recommending image fixes (no attribution data = guess, not audit)

Output: Performance Audit Report

## Performance Audit: [Project Name]

### Production Metrics (from Sentry)
- LCP: [value] — [good/needs work/poor]
- INP: [value] — [good/needs work/poor]
- CLS: [value] — [good/needs work/poor]
- Slowest transactions: [list with p95 times]

### Critical Issues (fix immediately)
| # | Area | Issue | Impact | Fix |
|---|------|-------|--------|-----|
| 1 | DB | N+1 on /users endpoint | p95 = 2.3s | Eager load orders relation |

### Optimizations (prioritized)
| # | Area | Issue | Impact | Effort | Fix |
|---|------|-------|--------|--------|-----|
| 1 | FE | Bundle 450KB gzipped | High | Low | Code split routes |

### Already Optimized
- [list of performance areas that are well-implemented]

### Monitoring Recommendations
- [what to add to track performance over time]

### Research Sources
- [URL] — [what optimization pattern it provided]