Core Web Vitals

Core Web Vitals

The rule: report measurements or report that you could not measure. A performance claim with no number attached is noise, and a number you did not observe is worse than no number.

Targets

Field thresholds (what Google grades on, 75th percentile of real users):

Lab numbers from Lighthouse on a dev build run hotter than field numbers. Treat a lab pass as necessary, not sufficient, and say which kind you measured.

Budget for a marketing or landing page, on top of the metrics:

• First-load JS for the route: ≤ 200 kB gzipped. Next.js prints this per route in the build output — that is a free measurement, always take it.
• Hero image: served in AVIF or WebP, sized to its container, priority set.
• Web fonts: at most two families, font-display: swap, preloaded if above the fold, subset to the characters actually used.

How to measure

Cheapest first. Stop when you have a number.

1. Build output (always available, zero setup).

# Next.js — the per-route First Load JS table is the budget check
npm run build 2>&1 | tail -40

Flag any route whose First Load JS grew in this change, and by how much.

2. Lighthouse against a running server.

npx --yes lighthouse http://localhost:3000/<route> \
  --only-categories=performance \
  --preset=desktop \
  --output=json --output-path=./.lighthouse.json --quiet --chrome-flags="--headless"
node -e "const r=require('./.lighthouse.json');const a=r.audits;console.log(
  'perf', Math.round(r.categories.performance.score*100),
  '| LCP', a['largest-contentful-paint'].displayValue,
  '| CLS', a['cumulative-layout-shift'].displayValue,
  '| TBT', a['total-blocking-time'].displayValue)"

Run mobile too (--preset= omitted defaults to mobile emulation) — mobile is where LCP actually fails.

3. In-page, when a browser is driveable (pairs with visual-proof):

new PerformanceObserver(l => console.log('LCP', l.getEntries().at(-1).startTime))
  .observe({ type: 'largest-contentful-paint', buffered: true });

If none of these are possible — no dev server, no network, no Chromium — say exactly that, list which of the static checks below you ran instead, and do not produce a score.

Static checks that need no browser

These are cheap greps and catch most real regressions:

• <img> without width/height → CLS risk
• next/image without priority on the LCP element, or with priority on below-fold images (which steals bandwidth from the real LCP)
• loading="lazy" missing on below-fold media
• a 'use client' boundary sitting higher in the tree than it needs to
• third-party <script> without strategy="lazyOnload" / defer
• font @import in CSS (blocking) instead of next/font or a preload link
• .map() over an unbounded array with no virtualization
• animation on width, height, top, left instead of transform/opacity
• missing <link rel="preconnect"> for an asset or media CDN

Reporting

Route:        /<path>
Measured by:  next build | lighthouse (desktop|mobile) | not measured — <why>
First Load:   NNN kB  (was NNN kB, delta +/-NN)
LCP:          N.Ns    CLS: N.NN    TBT/INP: NNNms
Static:       <findings, or clean>
Verdict:      within budget | over budget on <metric> | unmeasured

If the verdict is "over budget," name the single largest contributor before proposing fixes. Most pages have one dominant cause — an unoptimized hero, a font waterfall, or one fat client component — and fixing it beats five small optimizations.