| name | core-web-vitals |
| description | Optimize Core Web Vitals (LCP, INP, CLS) for better page experience and search ranking. Use when asked to "improve Core Web Vitals", "fix LCP", "reduce CLS", "optimize INP", "page experience optimization", or "fix layout shifts". Focuses specifically on the three Core Web Vitals metrics. Do NOT use for general web performance (use perf-web-optimization), Lighthouse audits (use perf-lighthouse), or Astro-specific optimization (use perf-astro). |
| license | MIT |
| metadata | author: web-quality-skills version: '1.0' |
Core Web Vitals optimization
Targeted optimization for the three Core Web Vitals metrics that affect Google Search ranking and user experience.
The three metrics
| Metric | Measures | Good | Needs work | Poor |
|---|
| LCP | Loading | ≤ 2.5s | 2.5s – 4s | > 4s |
| INP | Interactivity | ≤ 200ms | 200ms – 500ms | > 500ms |
| CLS | Visual Stability | ≤ 0.1 | 0.1 – 0.25 | > 0.25 |
Google measures at the 75th percentile — 75% of page visits must meet "Good" thresholds.
LCP: Largest Contentful Paint
LCP measures when the largest visible content element renders. Usually this is:
- Hero image or video
- Large text block
- Background image
<svg> element
Common LCP issues
1. Slow server response (TTFB > 800ms)
Fix: CDN, caching, optimized backend, edge rendering
2. Render-blocking resources
<link rel="stylesheet" href="/all-styles.css" />
<style>
</style>
<link rel="preload" href="/styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'" />
3. Slow resource load times
<img src="/hero.jpg" alt="Hero" />
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high" />
<img src="/hero.webp" alt="Hero" fetchpriority="high" />
4. Client-side rendering delays
useEffect(() => {
fetch('/api/hero-text')
.then((r) => r.json())
.then(setHeroText)
}, [])
export async function getServerSideProps() {
const heroText = await fetchHeroText()
return { props: { heroText } }
}
LCP optimization checklist
- [ ] TTFB < 800ms (use CDN, edge caching)
- [ ] LCP image preloaded with fetchpriority="high"
- [ ] LCP image optimized (WebP/AVIF, correct size)
- [ ] Critical CSS inlined (< 14KB)
- [ ] No render-blocking JavaScript in <head>
- [ ] Fonts don't block text rendering (font-display: swap)
- [ ] LCP element in initial HTML (not JS-rendered)
LCP element identification
new PerformanceObserver((list) => {
const entries = list.getEntries()
const lastEntry = entries[entries.length - 1]
console.log('LCP element:', lastEntry.element)
console.log('LCP time:', lastEntry.startTime)
}).observe({ type: 'largest-contentful-paint', buffered: true })
INP: Interaction to Next Paint
INP measures responsiveness across ALL interactions (clicks, taps, key presses) during a page visit. It reports the worst interaction (at 98th percentile for high-traffic pages).
INP breakdown
Total INP = Input Delay + Processing Time + Presentation Delay
| Phase | Target | Optimization |
|---|
| Input Delay | < 50ms | Reduce main thread blocking |
| Processing | < 100ms | Optimize event handlers |
| Presentation | < 50ms | Minimize rendering work |
Common INP issues
1. Long tasks blocking main thread
function processLargeArray(items) {
items.forEach((item) => expensiveOperation(item))
}
async function processLargeArray(items) {
const CHUNK_SIZE = 100
for (let i = 0; i < items.length; i += CHUNK_SIZE) {
const chunk = items.slice(i, i + CHUNK_SIZE)
chunk.forEach((item) => expensiveOperation(item))
await new Promise((r) => setTimeout(r, 0))
}
}
2. Heavy event handlers
button.addEventListener('click', () => {
const result = calculateComplexThing()
updateUI(result)
trackEvent('click')
})
button.addEventListener('click', () => {
button.classList.add('loading')
requestAnimationFrame(() => {
const result = calculateComplexThing()
updateUI(result)
})
requestIdleCallback(() => trackEvent('click'))
})
3. Third-party scripts
;<script src="https://heavy-widget.com/widget.js"></script>
const loadWidget = () => {
import('https://heavy-widget.com/widget.js').then((widget) => widget.init())
}
button.addEventListener('click', loadWidget, { once: true })
4. Excessive re-renders (React/Vue)
function App() {
const [count, setCount] = useState(0)
return (
<div>
<Counter count={count} />
<ExpensiveComponent /> {/* Re-renders on every count change */}
</div>
)
}
const MemoizedExpensive = React.memo(ExpensiveComponent)
function App() {
const [count, setCount] = useState(0)
return (
<div>
<Counter count={count} />
<MemoizedExpensive />
</div>
)
}
INP optimization checklist
- [ ] No tasks > 50ms on main thread
- [ ] Event handlers complete quickly (< 100ms)
- [ ] Visual feedback provided immediately
- [ ] Heavy work deferred with requestIdleCallback
- [ ] Third-party scripts don't block interactions
- [ ] Debounced input handlers where appropriate
- [ ] Web Workers for CPU-intensive operations
INP debugging
new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
if (entry.duration > 200) {
console.warn('Slow interaction:', {
type: entry.name,
duration: entry.duration,
processingStart: entry.processingStart,
processingEnd: entry.processingEnd,
target: entry.target,
})
}
}
}).observe({ type: 'event', buffered: true, durationThreshold: 16 })
CLS: Cumulative Layout Shift
CLS measures unexpected layout shifts. A shift occurs when a visible element changes position between frames without user interaction.
CLS Formula: impact fraction × distance fraction
Common CLS causes
1. Images without dimensions
<img src="photo.jpg" alt="Photo" />
<img src="photo.jpg" alt="Photo" width="800" height="600" />
<img src="photo.jpg" alt="Photo" style="aspect-ratio: 4/3; width: 100%;" />
2. Ads, embeds, and iframes
<iframe src="https://ad-network.com/ad"></iframe>
<div style="min-height: 250px;">
<iframe src="https://ad-network.com/ad" height="250"></iframe>
</div>
<div style="aspect-ratio: 16/9;">
<iframe src="https://youtube.com/embed/..." style="width: 100%; height: 100%;"></iframe>
</div>
3. Dynamically injected content
notifications.prepend(newNotification)
const insertBelow = viewport.bottom < newNotification.top
if (insertBelow) {
notifications.prepend(newNotification)
} else {
newNotification.style.transform = 'translateY(-100%)'
notifications.prepend(newNotification)
requestAnimationFrame(() => {
newNotification.style.transform = ''
})
}
4. Web fonts causing FOUT
@font-face {
font-family: 'Custom';
src: url('custom.woff2') format('woff2');
}
@font-face {
font-family: 'Custom';
src: url('custom.woff2') format('woff2');
font-display: optional;
}
@font-face {
font-family: 'Custom';
src: url('custom.woff2') format('woff2');
font-display: swap;
size-adjust: 105%;
ascent-override: 95%;
descent-override: 20%;
}
5. Animations triggering layout
.animate {
transition:
height 0.3s,
width 0.3s;
}
.animate {
transition: transform 0.3s;
}
.animate.expanded {
transform: scale(1.2);
}
CLS optimization checklist
- [ ] All images have width/height or aspect-ratio
- [ ] All videos/embeds have reserved space
- [ ] Ads have min-height containers
- [ ] Fonts use font-display: optional or matched metrics
- [ ] Dynamic content inserted below viewport
- [ ] Animations use transform/opacity only
- [ ] No content injected above existing content
CLS debugging
new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
if (!entry.hadRecentInput) {
console.log('Layout shift:', entry.value)
entry.sources?.forEach((source) => {
console.log(' Shifted element:', source.node)
console.log(' Previous rect:', source.previousRect)
console.log(' Current rect:', source.currentRect)
})
}
}
}).observe({ type: 'layout-shift', buffered: true })
Measurement tools
Lab testing
- Chrome DevTools → Performance panel, Lighthouse
- WebPageTest → Detailed waterfall, filmstrip
- Lighthouse CLI →
npx lighthouse <url>
Field data (real users)
- Chrome User Experience Report (CrUX) → BigQuery or API
- Search Console → Core Web Vitals report
- web-vitals library → Send to your analytics
import { onLCP, onINP, onCLS } from 'web-vitals'
function sendToAnalytics({ name, value, rating }) {
gtag('event', name, {
event_category: 'Web Vitals',
value: Math.round(name === 'CLS' ? value * 1000 : value),
event_label: rating,
})
}
onLCP(sendToAnalytics)
onINP(sendToAnalytics)
onCLS(sendToAnalytics)
Framework quick fixes
Next.js
import Image from 'next/image'
;<Image src="/hero.jpg" priority fill alt="Hero" />
const HeavyComponent = dynamic(() => import('./Heavy'), { ssr: false })
React
;<link rel="preload" href="/hero.jpg" as="image" fetchpriority="high" />
const [isPending, startTransition] = useTransition()
startTransition(() => setExpensiveState(newValue))
Vue/Nuxt
<!-- LCP: Use nuxt/image with preload -->
<NuxtImg src="/hero.jpg" preload loading="eager" />
<!-- INP: Use async components -->
<component :is="() => import('./Heavy.vue')" />
<!-- CLS: Use aspect-ratio CSS -->
<img :style="{ aspectRatio: '16/9' }" />
References
