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楊東霖

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Web Vitals Optimization: Achieve Core Web Vitals Score 90+ in 2026

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Web Vitals Optimization: Achieve Core Web Vitals Score 90+ in 2026

Core Web Vitals directly affect Google search rankings. A score under 50 hurts your SEO; above 90 gives you a ranking boost. Here's a systematic guide to achieving and maintaining 90+ scores.

The 2026 Core Web Vitals Metrics

Metric What It Measures Good Needs Work Poor
LCP (Largest Contentful Paint) Loading speed <2.5s 2.5-4s >4s
INP (Interaction to Next Paint) Responsiveness <200ms 200-500ms >500ms
CLS (Cumulative Layout Shift) Visual stability <0.1 0.1-0.25 >0.25

INP replaced FID (First Input Delay) in March 2024 and is the metric most developers underestimate.

Measure First

PageSpeed Insights (Field Data)

Test your real URLs at pagespeed.web.dev. It shows both lab data (Lighthouse) and field data (real user Chrome data). Field data is what Google uses for rankings.

Lighthouse CI in Your Pipeline 

# GitHub Actions: block deploys when vitals regress
- name: Run Lighthouse CI
  uses: treosh/lighthouse-ci-action@v11
  with:
    urls: |
      https://preview-${{ github.sha }}.your-app.com/
    budgetPath: .lighthouserc.json
    uploadArtifacts: true

# .lighthouserc.json
{
  "ci": {
    "assert": {
      "assertions": {
        "categories:performance": ["error", { "minScore": 0.9 }],
        "first-contentful-paint": ["error", { "maxNumericValue": 2000 }],
        "largest-contentful-paint": ["error", { "maxNumericValue": 2500 }],
        "cumulative-layout-shift": ["error", { "maxNumericValue": 0.1 }],
        "total-blocking-time": ["error", { "maxNumericValue": 300 }]
      }
    }
  }
}
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Real User Monitoring (RUM) 

// Measure CWV in production with the web-vitals library
import { onCLS, onINP, onLCP, onFCP, onTTFB } from 'web-vitals';

function sendToAnalytics(metric) {
  // Send to your analytics endpoint
  fetch('/api/vitals', {
    method: 'POST',
    body: JSON.stringify({
      name: metric.name,
      value: metric.value,
      rating: metric.rating, // 'good' | 'needs-improvement' | 'poor'
      delta: metric.delta,
      id: metric.id,
      navigationType: metric.navigationType,
    }),
    keepalive: true,
  });
}

onCLS(sendToAnalytics);
onINP(sendToAnalytics);
onLCP(sendToAnalytics);
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Optimizing LCP (Largest Contentful Paint)

LCP is usually your hero image, above-the-fold text, or a banner. The browser needs to discover, fetch, and render it quickly.

1. Preload Your LCP Element 

<!-- Tell the browser to fetch the LCP image immediately -->
<link rel="preload" as="image" href="/hero.webp"
      fetchpriority="high"
      imagesrcset="/hero-400.webp 400w, /hero-800.webp 800w, /hero-1200.webp 1200w"
      imagesizes="100vw">
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2. Use Modern Image Formats 

<!-- WebP with AVIF fallback — significant size reduction -->
<picture>
  <source type="image/avif" srcset="/hero.avif">
  <source type="image/webp" srcset="/hero.webp">
  <img src="/hero.jpg" alt="Hero" width="1200" height="600"
       fetchpriority="high">
</picture>
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AVIF achieves 50% better compression than WebP at equivalent quality. Use ImageMagick or Squoosh for conversion:

# Convert to AVIF (ImageMagick 7+)
magick hero.jpg -quality 80 hero.avif

# Convert to WebP
magick hero.jpg -quality 85 hero.webp
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3. Eliminate Render-Blocking Resources 

<!-- ❌ Render-blocking CSS in <head> -->
<link rel="stylesheet" href="/vendor.css">

<!-- ✅ Critical CSS inlined, rest deferred -->
<style>
  /* Critical above-the-fold styles inline */
  body { margin: 0; font-family: system-ui; }
  .hero { ... }
</style>
<link rel="stylesheet" href="/vendor.css" media="print" onload="this.media='all'">
<noscript><link rel="stylesheet" href="/vendor.css"></noscript>
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4. Use a CDN with Edge Caching

Without CDN: request travels datacenter → user (200-400ms added latency)
With CDN: cached at edge node near user (<50ms)

# Cache control for static assets
location ~* \.(webp|avif|woff2|js|css)$ {
  expires 1y;
  add_header Cache-Control "public, immutable";
}
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5. Server-Side Rendering for Text LCP

If your LCP element is text inside a JavaScript-rendered component, SSR it:

// Next.js: SSR the above-the-fold content
export async function getServerSideProps() {
  const heroData = await fetchHeroContent();
  return { props: { heroData } };
}

function HeroSection({ heroData }) {
  // Rendered on server → LCP text appears immediately
  return <h1>{heroData.headline}</h1>;
}
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Optimizing INP (Interaction to Next Paint)

INP measures how quickly your page responds to user interactions: clicks, taps, keyboard input. Unlike FID (which measured first interaction only), INP measures all interactions.

1. Break Up Long Tasks

The browser can't process user input during a long JavaScript task. Tasks over 50ms block the main thread.

// ❌ Long synchronous task blocks interactions
function processLargeDataset(items) {
  return items.map(item => expensiveTransform(item)); // 500ms+
}

// ✅ Yield to browser between chunks
async function processLargeDatasetAsync(items) {
  const results = [];
  const CHUNK_SIZE = 100;

  for (let i = 0; i < items.length; i += CHUNK_SIZE) {
    const chunk = items.slice(i, i + CHUNK_SIZE);
    results.push(...chunk.map(expensiveTransform));

    // Yield to browser: let it process pending interactions
    await new Promise(resolve => setTimeout(resolve, 0));
    // Or: await scheduler.yield(); (Chrome 115+)
  }

  return results;
}
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2. Use Scheduler.yield() for Smoother Interactions 

// Scheduler.yield: purpose-built for cooperative scheduling
async function handleButtonClick(event) {
  // First paint: show immediate feedback
  setLoading(true);

  // Yield: let browser update the UI before heavy work
  await scheduler.yield();

  // Now do expensive work
  const result = await processData(largeDataset);
  setResult(result);
  setLoading(false);
}
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3. Defer Non-Critical JavaScript 

// ❌ Import at top level: runs during initial parse
import { heavyAnalytics } from './analytics';
heavyAnalytics.init(); // Blocks first render

// ✅ Lazy load after user interaction
document.querySelector('#analytics-btn')?.addEventListener('click', async () => {
  const { heavyAnalytics } = await import('./analytics');
  heavyAnalytics.track('click');
});
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4. Optimize Event Handlers 

// ❌ Expensive work in input handler
input.addEventListener('input', (e) => {
  const results = searchItems(allItems, e.target.value); // Runs every keystroke
  renderResults(results);
});

// ✅ Debounce expensive operations
import { debounce } from './utils';

const debouncedSearch = debounce((query) => {
  const results = searchItems(allItems, query);
  renderResults(results);
}, 150);

input.addEventListener('input', (e) => {
  // Immediate feedback (input value update is instant)
  debouncedSearch(e.target.value); // Heavy work waits
});
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Optimizing CLS (Cumulative Layout Shift)

CLS penalizes unexpected layout shifts — elements moving around as the page loads. The most common causes:

1. Reserve Space for Images and Embeds 

<!-- ❌ No dimensions → layout shift when image loads -->
<img src="/product.jpg" alt="Product">

<!-- ✅ Width/height prevents layout shift -->
<img src="/product.jpg" alt="Product" width="800" height="600">

<!-- ✅ aspect-ratio in CSS for responsive images -->
<style>
  img {
    width: 100%;
    aspect-ratio: 4/3;
    height: auto;
  }
</style>
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2. Avoid Injecting Content Above Existing Content 

// ❌ Inserting banner above existing content
document.body.insertBefore(cookieBanner, document.body.firstChild);

// ✅ Use position: fixed or reserve space in layout
const cookieBanner = document.getElementById('cookie-banner');
// Already in HTML with reserved height, initially hidden
cookieBanner.style.display = 'block';
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3. Font Loading Optimization 

/* Prevent flash of invisible/unstyled text */
@font-face {
  font-family: 'CustomFont';
  src: url('/fonts/custom.woff2') format('woff2');
  font-display: swap;     /* Show system font → swap to custom font */
  font-display: optional; /* Use custom font only if cached — best for CLS */
}
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<!-- Preload critical fonts -->
<link rel="preload" href="/fonts/custom.woff2" as="font" type="font/woff2" crossorigin>
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4. Animations That Cause Layout Shifts 

/* ❌ These properties trigger layout recalculation */
.expanding {
  transition: height 0.3s; /* Causes layout shift */
  transition: margin 0.3s; /* Causes layout shift */
  transition: padding 0.3s; /* Causes layout shift */
}

/* ✅ Use transform and opacity — no layout recalculation */
.expanding {
  transition: transform 0.3s, opacity 0.3s;
  transform: scaleY(1);
}
.collapsed {
  transform: scaleY(0);
  opacity: 0;
}
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The 90+ Score Checklist

LCP < 2.5s

  • [ ] Preload LCP image with fetchpriority="high"
  • [ ] Serve images in WebP/AVIF format
  • [ ] Use a CDN with edge caching
  • [ ] Eliminate render-blocking CSS/JS
  • [ ] SSR above-the-fold content

INP < 200ms

  • [ ] No long tasks (>50ms) in JavaScript
  • [ ] Use scheduler.yield() or setTimeout(0) to yield
  • [ ] Debounce input handlers
  • [ ] Defer non-critical JavaScript loading
  • [ ] Code split routes and features

CLS < 0.1

  • [ ] All images have width and height attributes
  • [ ] Reserve space for dynamic content (ads, embeds)
  • [ ] Use font-display: optional for web fonts
  • [ ] Only animate transform and opacity
  • [ ] No content injected above existing content

Related Articles

Summary

Achieving 90+ Core Web Vitals requires attacking all three metrics simultaneously. LCP responds best to image optimization, CDN deployment, and eliminating render-blocking resources. INP requires breaking up long JavaScript tasks and deferring non-critical work. CLS is fixed by reserving space for dynamic content and using transform-based animations.

Set up Lighthouse CI in your pipeline to catch regressions before they reach production. Add real user monitoring with the web-vitals library to track actual user experience, not just lab conditions. The gap between lab and field data is often 20-40% — optimizing for real users is what moves the needle on search rankings.

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