Core Web Vitals optimization

Overview

This public intake copy packages

packages/skills-catalog/skills/(performance)/core-web-vitals

https://github.com/tech-leads-club/agent-skills

Use it when the operator needs the upstream workflow, support files, and repository context to stay intact while the public validator and private enhancer continue their normal downstream flow.

This intake keeps the copied upstream files intact and uses

metadata.json

ORIGIN.md

Core Web Vitals optimization Targeted optimization for the three Core Web Vitals metrics that affect Google Search ranking and user experience.

Imported source sections that did not map cleanly to the public headings are still preserved below or in the support files. Notable imported sections: The three metrics, LCP: Largest Contentful Paint, INP: Interaction to Next Paint, CLS: Cumulative Layout Shift, Measurement tools, Framework quick fixes.

When to Use This Skill

Use this section as the trigger filter. It should make the activation boundary explicit before the operator loads files, runs commands, or opens a pull request.

• Use when the request clearly matches the imported source intent: 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....
• Use when the operator should preserve upstream workflow detail instead of rewriting the process from scratch.
• Use when provenance needs to stay visible in the answer, PR, or review packet.
• Use when copied upstream references, examples, or scripts materially improve the answer.
• Use when the workflow should remain reviewable in the public intake repo before the private enhancer takes over.

Operating Table

metadata.json

ORIGIN.md

references/LCP.md

references/LCP.md

## Related Skills

Workflow

This workflow is intentionally editorial and operational at the same time. It keeps the imported source useful to the operator while still satisfying the public intake standards that feed the downstream enhancer flow.

1. Confirm the user goal, the scope of the imported workflow, and whether this skill is still the right router for the task.
2. Read the overview and provenance files before loading any copied upstream support files.
3. Load only the references, examples, prompts, or scripts that materially change the outcome for the current request.
4. Execute the upstream workflow while keeping provenance and source boundaries explicit in the working notes.
5. Validate the result against the upstream expectations and the evidence you can point to in the copied files.
6. Escalate or hand off to a related skill when the work moves out of this imported workflow's center of gravity.
7. Before merge or closure, record what was used, what changed, and what the reviewer still needs to verify.

Imported Workflow Notes

Imported: The three metrics

Google measures at the 75th percentile — 75% of page visits must meet "Good" thresholds.

Examples

Example 1: Ask for the upstream workflow directly

Use @core-web-vitals to handle <task>. Start from the copied upstream workflow, load only the files that change the outcome, and keep provenance visible in the answer.

Explanation: This is the safest starting point when the operator needs the imported workflow, but not the entire repository.

Example 2: Ask for a provenance-grounded review

Review @core-web-vitals against metadata.json and ORIGIN.md, then explain which copied upstream files you would load first and why.

Explanation: Use this before review or troubleshooting when you need a precise, auditable explanation of origin and file selection.

Example 3: Narrow the copied support files before execution

Use @core-web-vitals for <task>. Load only the copied references, examples, or scripts that change the outcome, and name the files explicitly before proceeding.

Explanation: This keeps the skill aligned with progressive disclosure instead of loading the whole copied package by default.

Example 4: Build a reviewer packet

Review @core-web-vitals using the copied upstream files plus provenance, then summarize any gaps before merge.

Explanation: This is useful when the PR is waiting for human review and you want a repeatable audit packet.

Best Practices

Treat the generated public skill as a reviewable packaging layer around the upstream repository. The goal is to keep provenance explicit and load only the copied source material that materially improves execution.

• Keep the imported skill grounded in the upstream repository; do not invent steps that the source material cannot support.
• Prefer the smallest useful set of support files so the workflow stays auditable and fast to review.
• Keep provenance, source commit, and imported file paths visible in notes and PR descriptions.
• Point directly at the copied upstream files that justify the workflow instead of relying on generic review boilerplate.
• Treat generated examples as scaffolding; adapt them to the concrete task before execution.
• Route to a stronger native skill when architecture, debugging, design, or security concerns become dominant.

Troubleshooting

Problem: The operator skipped the imported context and answered too generically

Symptoms: The result ignores the upstream workflow in

packages/skills-catalog/skills/(performance)/core-web-vitals

metadata.json

ORIGIN.md

Problem: The imported workflow feels incomplete during review

Symptoms: Reviewers can see the generated

SKILL.md

Problem: The task drifted into a different specialization

Symptoms: The imported skill starts in the right place, but the work turns into debugging, architecture, design, security, or release orchestration that a native skill handles better. Solution: Use the related skills section to hand off deliberately. Keep the imported provenance visible so the next skill inherits the right context instead of starting blind.

Related Skills

• @accessibility

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @ai-cold-outreach

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @ai-pricing

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

• @ai-sdr

  - Use when the work is better handled by that native specialization after this imported skill establishes context.

Additional Resources

Use this support matrix and the linked files below as the operator packet for this imported skill. They should reflect real copied source material, not generic scaffolding.

references

references/LCP.md

examples

examples/n/a

scripts

scripts/n/a

agents

agents/n/a

assets

assets/n/a

• LCP.md
• LCP.md

Imported Reference Notes

Imported: References

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

Imported: 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

<!-- ❌ 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

<!-- ❌ 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

// ❌ 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

- [ ] 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

// 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 })

Imported: 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

Common INP issues

1. Long tasks blocking main thread

// ❌ 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

// ❌ 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

// ❌ 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)

// ❌ 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

- [ ] 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

// 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 })

Imported: 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

<!-- ❌ 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

<!-- ❌ 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

// ❌ 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

/* ❌ 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

/* ❌ 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

- [ ] 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

// 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 })

Imported: 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)

Imported: Framework quick fixes

Next.js

// 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

// 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

<!-- 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' }" />