core-web-vitals▌
tech-leads-club/agent-skills · updated May 23, 2026
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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).
| 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
<!-- ❌ 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 })
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
// ❌ 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 })
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 })
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
// 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' }" />
References
How to use core-web-vitals on Cursor
AI-first code editor with Composer
Prerequisites
Before installing skills in Cursor, ensure your development environment meets these requirements:
- ›Cursor installed and configured on your development machine
- ›Node.js version 16.0+ with npm package manager (verify with
node --version) - ›Active project directory or workspace where you want to add core-web-vitals
Execute installation command
Execute the skills CLI command in your project's root directory to begin installation:
The skills CLI fetches core-web-vitals from GitHub repository tech-leads-club/agent-skills and configures it for Cursor.
Select Cursor when prompted
The CLI will show a list of available agents. Use arrow keys to navigate and space to select Cursor:
Verify installation
Confirm successful installation by checking the skill directory location:
Reload or restart Cursor to activate core-web-vitals. Access the skill through slash commands (e.g., /core-web-vitals) or your agent's skill management interface.
Security & Verification Notice
We perform automated surface-level scans (Gen AI Scanner, Socket, Snyk) during installation. These checks detect common vulnerabilities but do not guarantee complete security. Always review skill source code and verify the publisher's reputation before production use.
Skills execute code in your development environment. Always verify the publisher's identity, review recent commits, and test in isolated environments before production deployment.
List & Monetize Your Skill
Submit your Claude Code skill and start earning
Use Cases▌
Task Automation & Efficiency
Automate repetitive workflows and reduce manual effort
Example
Generate reports, summarize documents, draft communications
Save 3-5 hours per week on routine tasks
Knowledge Enhancement
Learn new skills, understand complex topics, get expert guidance
Example
Explain concepts, provide examples, suggest learning resources
Accelerate learning and skill development by 2x
Quality Improvement
Enhance output quality through reviews, suggestions, and refinements
Example
Review drafts, suggest improvements, catch errors
Improve work quality by 30-40% with less effort
Implementation Guide▌
Prerequisites
- ›Claude Desktop or compatible AI client with skill support
- ›Clear understanding of task or problem to solve
- ›Willingness to iterate and refine outputs
Time Estimate
15-45 minutes depending on use case complexity
Installation Steps
- 1.Install skill using provided installation command
- 2.Test with simple use case relevant to your work
- 3.Evaluate output quality and relevance
- 4.Iterate on prompts to improve results
- 5.Integrate into regular workflow if valuable
Common Pitfalls
- ⚠Expecting perfect results without iteration
- ⚠Not providing enough context in prompts
- ⚠Using skill for tasks outside its intended scope
- ⚠Accepting outputs without review and validation
Best Practices▌
✓ Do
- +Start with clear, specific prompts
- +Provide relevant context and constraints
- +Review and refine all outputs before using
- +Iterate to improve output quality
- +Document successful prompt patterns
✗ Don't
- −Don't use without understanding skill limitations
- −Don't skip validation of outputs
- −Don't share sensitive information in prompts
- −Don't expect skill to replace human judgment
💡 Pro Tips
- ★Be specific about desired format and style
- ★Ask for multiple options to choose from
- ★Request explanations to understand reasoning
- ★Combine AI efficiency with human expertise
When to Use This▌
✓ Use When
Use when skill capabilities match your task, clear ROI on time saved, and you can validate outputs. Best for repetitive tasks, learning, and quality improvement.
✗ Avoid When
Avoid when task requires deep expertise you can't validate, involves sensitive decisions, or when learning process is more valuable than speed of completion.
Learning Path▌
- 1Familiarize yourself with skill capabilities and limitations
- 2Start with low-risk, non-critical tasks
- 3Progress to more complex and valuable use cases
- 4Build expertise through regular use and experimentation
Discussion
Product Hunt–style comments (not star reviews)- No comments yet — start the thread.
Ratings
4.7★★★★★25 reviews- ★★★★★Pratham Ware· Dec 28, 2024
Solid pick for teams standardizing on skills: core-web-vitals is focused, and the summary matches what you get after install.
- ★★★★★Kaira Gonzalez· Dec 28, 2024
core-web-vitals fits our agent workflows well — practical, well scoped, and easy to wire into existing repos.
- ★★★★★Kiara Reddy· Dec 20, 2024
core-web-vitals has been reliable in day-to-day use. Documentation quality is above average for community skills.
- ★★★★★Nikhil Park· Dec 12, 2024
I recommend core-web-vitals for anyone iterating fast on agent tooling; clear intent and a small, reviewable surface area.
- ★★★★★Ama Mehta· Nov 19, 2024
Registry listing for core-web-vitals matched our evaluation — installs cleanly and behaves as described in the markdown.
- ★★★★★Hana Wang· Nov 3, 2024
Keeps context tight: core-web-vitals is the kind of skill you can hand to a new teammate without a long onboarding doc.
- ★★★★★Carlos Okafor· Oct 22, 2024
core-web-vitals is among the better-maintained entries we tried; worth keeping pinned for repeat workflows.
- ★★★★★Ama Ghosh· Oct 10, 2024
core-web-vitals reduced setup friction for our internal harness; good balance of opinion and flexibility.
- ★★★★★Ama Gill· Sep 13, 2024
core-web-vitals fits our agent workflows well — practical, well scoped, and easy to wire into existing repos.
- ★★★★★Sakshi Patil· Sep 5, 2024
I recommend core-web-vitals for anyone iterating fast on agent tooling; clear intent and a small, reviewable surface area.
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