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

html

<!-- ❌ Blocks rendering -->
<link rel="stylesheet" href="/all-styles.css">

<!-- ✅ Critical CSS inlined, rest deferred -->
<style>/* Critical above-fold CSS */</style>
<link rel="preload" href="/styles.css" as="style" 
      onload="this.onload=null;this.rel='stylesheet'">

3. Slow resource load times

html

<!-- ❌ No hints, discovered late -->
<img src="/hero.jpg" alt="Hero">

<!-- ✅ Preloaded with high priority -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high">
<img src="/hero.webp" alt="Hero" fetchpriority="high">

4. Client-side rendering delays

javascript

// ❌ Content loads after JavaScript
useEffect(() => {
  fetch('/api/hero-text').then(r => r.json()).then(setHeroText);
}, []);

// ✅ Server-side or static rendering
// Use SSR, SSG, or streaming to send HTML with content
export async function getServerSideProps() {
  const heroText = await fetchHeroText();
  return { props: { heroText } };
}

LCP optimization checklist

markdown

- [ ] 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

javascript

// Find your LCP element
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

javascript

// ❌ Long synchronous task
function processLargeArray(items) {
  items.forEach(item => expensiveOperation(item));
}

// ✅ Break into chunks with yielding
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));
    
    // Yield to main thread
    await new Promise(r => setTimeout(r, 0));
    // Or use scheduler.yield() when available
  }
}

2. Heavy event handlers

javascript

// ❌ All work in handler
button.addEventListener('click', () => {
  // Heavy computation
  const result = calculateComplexThing();
  // DOM updates
  updateUI(result);
  // Analytics
  trackEvent('click');
});

// ✅ Prioritize visual feedback
button.addEventListener('click', () => {
  // Immediate visual feedback
  button.classList.add('loading');
  
  // Defer non-critical work
  requestAnimationFrame(() => {
    const result = calculateComplexThing();
    updateUI(result);
  });
  
  // Use requestIdleCallback for analytics
  requestIdleCallback(() => trackEvent('click'));
});

3. Third-party scripts

javascript

// ❌ Eagerly loaded, blocks interactions
<script src="https://heavy-widget.com/widget.js"></script>

// ✅ Lazy loaded on interaction or visibility
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)

javascript

// ❌ Re-renders entire tree
function App() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <Counter count={count} />
      <ExpensiveComponent /> {/* Re-renders on every count change */}
    </div>
  );
}

// ✅ Memoized expensive components
const MemoizedExpensive = React.memo(ExpensiveComponent);

function App() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <Counter count={count} />
      <MemoizedExpensive />
    </div>
  );
}

INP optimization checklist

markdown

- [ ] 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

javascript

// Identify slow interactions
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

html

<!-- ❌ Causes layout shift when loaded -->
<img src="photo.jpg" alt="Photo">

<!-- ✅ Space reserved -->
<img src="photo.jpg" alt="Photo" width="800" height="600">

<!-- ✅ Or use aspect-ratio -->
<img src="photo.jpg" alt="Photo" style="aspect-ratio: 4/3; width: 100%;">

2. Ads, embeds, and iframes

html

<!-- ❌ Unknown size until loaded -->
<iframe src="https://ad-network.com/ad"></iframe>

<!-- ✅ Reserve space with min-height -->
<div style="min-height: 250px;">
  <iframe src="https://ad-network.com/ad" height="250"></iframe>
</div>

<!-- ✅ Or use aspect-ratio container -->
<div style="aspect-ratio: 16/9;">
  <iframe src="https://youtube.com/embed/..." 
          style="width: 100%; height: 100%;"></iframe>
</div>

3. Dynamically injected content

javascript

// ❌ Inserts content above viewport
notifications.prepend(newNotification);

// ✅ Insert below viewport or use transform
const insertBelow = viewport.bottom < newNotification.top;
if (insertBelow) {
  notifications.prepend(newNotification);
} else {
  // Animate in without shifting
  newNotification.style.transform = 'translateY(-100%)';
  notifications.prepend(newNotification);
  requestAnimationFrame(() => {
    newNotification.style.transform = '';
  });
}

4. Web fonts causing FOUT

css

/* ❌ Font swap shifts text */
@font-face {
  font-family: 'Custom';
  src: url('custom.woff2') format('woff2');
}

/* ✅ Optional font (no shift if slow) */
@font-face {
  font-family: 'Custom';
  src: url('custom.woff2') format('woff2');
  font-display: optional;
}

/* ✅ Or match fallback metrics */
@font-face {
  font-family: 'Custom';
  src: url('custom.woff2') format('woff2');
  font-display: swap;
  size-adjust: 105%; /* Match fallback size */
  ascent-override: 95%;
  descent-override: 20%;
}

5. Animations triggering layout

css

/* ❌ Animates layout properties */
.animate {
  transition: height 0.3s, width 0.3s;
}

/* ✅ Use transform instead */
.animate {
  transition: transform 0.3s;
}
.animate.expanded {
  transform: scale(1.2);
}

CLS optimization checklist

markdown

- [ ] 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

javascript

// Track layout shifts
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

javascript

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

jsx

// LCP: Use next/image with priority
import Image from 'next/image';
<Image src="/hero.jpg" priority fill alt="Hero" />

// INP: Use dynamic imports
const HeavyComponent = dynamic(() => import('./Heavy'), { ssr: false });

// CLS: Image component handles dimensions automatically

React

jsx

// LCP: Preload in head
<link rel="preload" href="/hero.jpg" as="image" fetchpriority="high" />

// INP: Memoize and useTransition
const [isPending, startTransition] = useTransition();
startTransition(() => setExpensiveState(newValue));

// CLS: Always specify dimensions in img tags

Vue/Nuxt

vue

<!-- 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

web.dev LCP
web.dev INP
web.dev CLS
Performance skill

core-web-vitals

NPX Install

SKILL.md Content

Core Web Vitals optimization

The three metrics

LCP: Largest Contentful Paint

Common LCP issues

LCP optimization checklist

LCP element identification

INP: Interaction to Next Paint

INP breakdown

Common INP issues

INP optimization checklist

INP debugging

CLS: Cumulative Layout Shift

Common CLS causes

CLS optimization checklist

CLS debugging

Measurement tools

Lab testing

Field data (real users)

Framework quick fixes

Next.js

React

Vue/Nuxt

References