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Core Web Vitals monitoring (LCP, FID, CLS, INP, TTFB), measurement with web-vitals library, reporting to analytics, and optimization strategies for Next.js
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インストールを決める前に、SKILL.md と SkillsMP に表示されている付属ファイルをお読みください。
Codex または Claude でインストール この Prompt をコピーして Codex、Claude、または他のアシスタントに貼り付けると、Skill ページを確認してインストールできます。
直接コマンドでは確認用 Prompt が省略されます。実行前にソースを確認してください。
npx skills add https://github.com/InugamiDev/ultrathink-oss --skill web-vitalsコマンドは1行のまま表示されます。コピー前に横へスクロールして全体を確認してください。
ローカルで確認しますか？SkillsMP が現在取得できるファイルをダウンロードできます。
SKILL.md を表示中
| name | web-vitals |
| description | Core Web Vitals monitoring (LCP, FID, CLS, INP, TTFB), measurement with web-vitals library, reporting to analytics, and optimization strategies for Next.js |
| layer | domain |
| category | performance |
| triggers | ["web vitals","core web vitals","LCP","CLS","INP","TTFB","FID","page speed","lighthouse","performance score","largest contentful paint","cumulative layout shift","interaction to next paint"] |
| inputs | ["Application URL or codebase","Current performance metrics or Lighthouse scores","Target performance budgets"] |
| outputs | ["Web Vitals measurement setup","Performance optimization recommendations","Reporting pipeline configuration","Next.js-specific optimizations"] |
| linksTo | ["nextjs","performance-profiler","monitoring","image-optimization","chrome-devtools"] |
| linkedFrom | ["optimize","audit","seo"] |
| preferredNextSkills | ["performance-profiler","image-optimization","nextjs"] |
| fallbackSkills | ["optimize","chrome-devtools"] |
| riskLevel | low |
| memoryReadPolicy | selective |
| memoryWritePolicy | none |
| sideEffects | ["May add web-vitals dependency","May modify Next.js configuration","May add reporting endpoints"] |
Measure, monitor, and optimize Core Web Vitals -- the user-centric performance metrics that Google uses for search ranking. This skill covers the web-vitals library, real-user monitoring (RUM), lab testing, and concrete optimization strategies for each metric.
| Metric | Full Name | Good | Needs Improvement | Poor | What It Measures |
|---|---|---|---|---|---|
| LCP | Largest Contentful Paint | <= 2.5s | <= 4.0s | > 4.0s | Loading performance |
| INP | Interaction to Next Paint | <= 200ms | <= 500ms | > 500ms | Responsiveness (replaced FID) |
| CLS | Cumulative Layout Shift | <= 0.1 | <= 0.25 | > 0.25 | Visual stability |
| Metric | Full Name | Good | What It Measures |
|---|---|---|---|
| TTFB | Time to First Byte | <= 800ms | Server responsiveness |
| FCP | First Contentful Paint | <= 1.8s | Perceived load speed |
| FID | First Input Delay (deprecated) | <= 100ms | Legacy responsiveness metric |
// lib/web-vitals.ts
import { onLCP, onINP, onCLS, onFCP, onTTFB, type Metric } from "web-vitals";
type ReportHandler = (metric: Metric) => void;
: = {
body = {
: metric.,
: metric.,
: metric.,
: metric.,
: metric.,
: metric.,
: metric..( ({
: e.,
: e.,
: e ? e. : ,
})),
};
(navigator.) {
navigator.(, .(body));
} {
(, {
: ,
: .(body),
: ,
});
}
};
() {
(sendToAnalytics);
(sendToAnalytics);
(sendToAnalytics);
(sendToAnalytics);
(sendToAnalytics);
}
// app/layout.tsx
import { WebVitalsReporter } from "@/components/web-vitals-reporter";
export default function RootLayout({ children }: { children: React.ReactNode }) {
return (
<html lang="en">
<body>
{children}
<WebVitalsReporter />
</body>
</html>
);
}
// components/web-vitals-reporter.tsx
"use client";
import { useEffect } from "react";
import { reportWebVitals } from "@/lib/web-vitals";
export function WebVitalsReporter() {
useEffect(() => {
reportWebVitals();
}, []);
return null;
}
// app/api/vitals/route.ts
import { NextRequest, NextResponse } from "next/server";
export async function POST(request: NextRequest) {
const metric = await request.json();
// Log for monitoring (replace with your analytics pipeline)
console.log(`[Web Vital] ${metric.name}: ${metric.value} (${metric.rating})`);
// Forward to your analytics service
// await analytics.track("web_vital", metric);
return NextResponse.json({ ok: true });
}
// Priority hints for LCP element
// In Next.js, use priority prop on the LCP image
import Image from "next/image";
export function HeroSection() {
return (
<section>
{/* priority prop adds fetchpriority="high" and preloads the image */}
<Image
src="/hero.webp"
alt="Hero banner"
width={1200}
height={600}
priority
sizes="100vw"
/>
<h1>Welcome</h1>
</section>
);
}
<!-- Manual preload for critical resources -->
<head>
<!-- Preload LCP image -->
<link rel="preload" as="image" href="/hero.webp" fetchpriority="high" />
<!-- Preload critical fonts -->
<link rel="preload" as="font" href="/fonts/inter-var.woff2"
type="font/woff2" crossorigin="anonymous" />
<!-- Preconnect to critical origins -->
<link rel="preconnect" href="https://cdn.example.com" />
<link rel="dns-prefetch" href="https://api.example.com" />
</head>
// next.config.ts -- optimize images for LCP
import type { NextConfig } from "next";
const nextConfig: NextConfig = {
images: {
formats: ["image/avif", "image/webp"],
deviceSizes: [640, 750, 828, 1080, 1200, 1920, 2048],
minimumCacheTTL: 31536000, // 1 year
},
experimental: {
optimizeCss: true, // Minimize CSS for faster rendering
optimizePackageImports: [
"lucide-react",
"@radix-ui/react-icons",
],
},
};
export default nextConfig;
// Always set explicit dimensions on images and videos
// BAD: causes layout shift
<img src="/photo.jpg" alt="Photo" />
// GOOD: reserves space
<img src="/photo.jpg" alt="Photo" width={800} height={600} />
// Next.js Image handles this automatically
<Image src="/photo.jpg" alt="Photo" width={800} height={600} />
/* Reserve space for dynamic content with aspect-ratio */
.video-container {
aspect-ratio: 16 / 9;
width: 100%;
background-color: #f0f0f0; /* placeholder color */
}
/* Prevent font swap layout shifts */
@font-face {
font-family: "Inter";
src: url("/fonts/inter-var.woff2") format("woff2");
font-display: optional; /* or "swap" with size-adjust */
size-adjust: 100%;
ascent-override: 90%;
descent-override: 20%;
line-gap-override: 0%;
}
/* Prevent ad/embed CLS */
.ad-slot {
min-height: 250px; /* reserve minimum space */
contain: layout; /* CSS containment */
}
// Prevent CLS from dynamically loaded content
"use client";
import { useState, useTransition } from "react";
export function DynamicList() {
const [items, setItems] = useState<Item[]>([]);
const [isPending, startTransition] = useTransition();
const loadMore = () => {
startTransition(async () => {
const newItems = await fetchItems();
setItems((prev) => [...prev, ...newItems]);
});
};
return (
<div>
{/* Fixed-height container prevents CLS */}
<div style={{ minHeight: items.length > 0 ? "auto" : "400px" }}>
{items.map((item) => (
<div key={item.id} className="h-20"> {/* fixed item height */}
{item.name}
</div>
))}
</div>
<button onClick={loadMore} disabled={isPending}>
{isPending ? "Loading..." : "Load More"}
</button>
</div>
);
}
// Break up long tasks with yield-to-main patterns
function yieldToMain(): Promise<void> {
return new Promise((resolve) => {
// scheduler.yield() is the modern API (Chrome 129+)
if ("scheduler" in globalThis && "yield" in (globalThis as any).scheduler) {
(globalThis as any).scheduler.yield().then(resolve);
} else {
setTimeout(resolve, 0);
}
});
}
// Use in long-running event handlers
async function handleExpensiveClick(items: Item[]) {
for (let i = 0; i < items.length; i++) {
processItem(items[i]);
// Yield every 50 items to keep INP low
if (i % 50 === 0 && i > 0) {
await yieldToMain();
}
}
}
// Use React transitions for non-urgent updates
"use client";
import { useState, useTransition, useDeferredValue } from "react";
export function SearchWithResults() {
const [query, setQuery] = useState("");
const deferredQuery = useDeferredValue(query);
const [isPending, startTransition] = useTransition();
const handleInput = (e: React.ChangeEvent<HTMLInputElement>) => {
// Urgent: update input value immediately
setQuery(e.target.value);
// Results will re-render with deferred value (non-blocking)
};
return (
<div>
<input value={query} onChange={handleInput} />
<div style={{ opacity: isPending ? 0.7 : 1 }}>
<SearchResults query={deferredQuery} />
</div>
</div>
);
}
// next.config.ts -- enable streaming and edge rendering where appropriate
const nextConfig: NextConfig = {
// Use PPR (Partial Prerendering) for optimal TTFB
experimental: {
ppr: true,
},
};
// app/products/page.tsx -- streaming with Suspense
import { Suspense } from "react";
export default function ProductsPage() {
return (
<div>
{/* Shell renders immediately (fast TTFB) */}
<h1>Products</h1>
<SearchBar />
{/* Data streams in as it resolves */}
<Suspense fallback={<ProductGridSkeleton />}>
<ProductGrid />
</Suspense>
<Suspense fallback={<RecommendationsSkeleton />}>
<Recommendations />
</Suspense>
</div>
);
}
// performance-budget.config.ts
export const performanceBudget = {
lcp: 2500, // ms
inp: 200, // ms
cls: 0.1, // score
ttfb: 800, // ms
fcp: 1800, // ms
// Bundle budgets
jsBundle: 150_000, // 150KB compressed
cssBundle: 50_000, // 50KB compressed
totalTransfer: 500_000, // 500KB total
} as const;
// CI check with Lighthouse
// .github/workflows/lighthouse.yml
/*
name: Lighthouse CI
on: [pull_request]
jobs:
lighthouse:
runs-on: ubuntu-latest
steps:
- uses: actions/checkout@v4
- uses: actions/setup-node@v4
- run: npm ci && npm run build
- name: Lighthouse
uses: treosh/lighthouse-ci-action@v12
with:
urls: |
http://localhost:3000/
http://localhost:3000/products
budgetPath: ./budget.json
uploadArtifacts: true
*/
// budget.json (Lighthouse CI budget)
[
{
"path": "/*",
"timings": [
{ "metric": "largest-contentful-paint", "budget": 2500 },
{ "metric": "cumulative-layout-shift", "budget": 0.1 },
{ "metric": "interactive", "budget": 3500 },
{ "metric": "first-contentful-paint", "budget": 1800 }
],
"resourceSizes": [
{ "resourceType": "script", "budget": 150 },
{ "resourceType": "stylesheet", "budget": 50 },
{ "resourceType": "total", "budget": 500 }
]
}
]
web-vitals reflects real user experiencefetchpriority="high", preload, and avoid lazy-loading the LCP elementfont-display: optional or swap with proper fallback metricsnext/dynamic, or <script defer> for non-essential codecontain: layout prevents layout recalculations from propagating| Pitfall | Impact | Fix |
|---|---|---|
| Lazy-loading the LCP image | LCP delayed by intersection observer | Use priority prop or fetchpriority="high" |
| Injecting content above the fold after load | CLS spike | Reserve space or use CSS contain |
| Large synchronous event handlers | Poor INP | Break up work, use startTransition, yield to main |
| Third-party scripts blocking render | LCP and INP degradation | Load with async/defer, use Partytown for heavy scripts |
| No font fallback metrics | CLS on font swap | Use size-adjust, ascent-override in @font-face |
| Measuring only in lab (Lighthouse) | Misses real-world variation | Set up RUM with web-vitals library |
| Not segmenting by page type | Aggregate data hides problems | Report vitals per route/page template |
| Render-blocking CSS | Slow FCP and LCP | Inline critical CSS, defer non-critical stylesheets |
1. Install: npm install web-vitals
2. Create lib/web-vitals.ts with measurement + sendBeacon reporting
3. Create WebVitalsReporter client component, mount in root layout
4. Create /api/vitals endpoint to receive and forward metrics
5. Set up Lighthouse CI in GitHub Actions for PR checks
6. Configure performance budgets in budget.json
7. Monitor field data in analytics dashboard
Problem: LCP = 4.2s, CLS = 0.35, INP = 450ms
LCP Fix:
- Move hero image from lazy to priority loading
- Preconnect to image CDN origin
- Convert from PNG to AVIF/WebP
CLS Fix:
- Add width/height to all product images
- Reserve space for price badge that loads async
- Set font-display: optional on custom font
INP Fix:
- Wrap filter state update in startTransition
- Virtualize product grid (only render visible items)
- Move sort computation to Web Worker
Result: LCP = 1.8s, CLS = 0.02, INP = 120ms
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