Web Performance & Core Web Vitals Expert

English | Bahasa Indonesia

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<a name="english"></a>

English

Orchestration & Integration

Connects and orchestrates with relevant domain skills like brainstorming, zero-to-prod-orchestrator, and session-memory-manager to ensure cohesive execution.

Description

Expert guide for measuring, diagnosing, and optimizing web application performance with a focus on Core Web Vitals (LCP, INP, CLS), JavaScript bundle optimization, image and font loading strategies, rendering performance, and achieving high Lighthouse scores. Covers Next.js 15, React 19, and modern browser APIs.

Trigger Conditions

• Improving Lighthouse performance score below 90.
• Diagnosing poor LCP, INP, or CLS metrics.
• Reducing JavaScript bundle size or eliminating render-blocking resources.
• Optimizing image loading, lazy loading, or font delivery.
• Implementing React performance patterns (memo, useDeferredValue, Suspense).
• Setting up Real User Monitoring (RUM) for Core Web Vitals.
• Optimizing server response times (TTFB).

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Core Web Vitals Targets (2026 Standards)

Metric	Good	Needs Improvement	Poor	Measures
LCP (Largest Contentful Paint)	≤ 2.5s	2.5–4.0s	> 4.0s	Loading performance
INP (Interaction to Next Paint)	≤ 200ms	200–500ms	> 500ms	Interactivity (replaced FID)
CLS (Cumulative Layout Shift)	≤ 0.1	0.1–0.25	> 0.25	Visual stability
FCP (First Contentful Paint)	≤ 1.8s	1.8–3.0s	> 3.0s	Perceived loading
TTFB (Time to First Byte)	≤ 800ms	800–1800ms	> 1800ms	Server responsiveness

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LCP Optimization Strategies

1. Identify and Optimize the LCP Element

// Measure LCP with web-vitals library
import { onLCP, onINP, onCLS } from 'web-vitals';

onLCP((metric) => {
  console.log('LCP:', metric.value, 'element:', metric.attribution.lcpEntry?.element);
  // Send to analytics
  sendToAnalytics({ name: metric.name, value: metric.value });
});

2. Preload LCP Image (Hero Image)

<!-- In <head> — preload the hero image before CSS blocks it -->
<link rel="preload" as="image" href="/hero.webp"
      imagesrcset="/hero-480.webp 480w, /hero-800.webp 800w"
      imagesizes="(max-width: 600px) 480px, 800px"
      fetchpriority="high">

3. Next.js Image Component (LCP Hero)

import Image from 'next/image';

// LCP image: priority=true + fill or explicit size
<Image
  src="/hero.webp"
  alt="Hero banner"
  width={1200}
  height={600}
  priority           // disables lazy loading, adds preload link
  fetchPriority="high"
  quality={85}
  sizes="(max-width: 768px) 100vw, 1200px"
/>

// Below-the-fold images: lazy load (default)
<Image
  src="/product.webp"
  alt="Product"
  width={400}
  height={300}
  // loading="lazy" is the default — no need to specify
/>

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INP Optimization Strategies

1. Break Up Long Tasks

// BAD: Long synchronous task blocks the main thread
function processLargeList(items: Item[]) {
  return items.map(expensiveTransform); // blocks for 500ms
}

// GOOD: Yield control back to browser between chunks
async function processLargeListAsync(items: Item[]) {
  const results: Result[] = [];
  for (let i = 0; i < items.length; i++) {
    results.push(expensiveTransform(items[i]));
    if (i % 50 === 0) await scheduler.yield(); // yield every 50 items
  }
  return results;
}

2. React 19 — Concurrent Features for INP

import { useTransition, useDeferredValue, startTransition } from 'react';

// Non-urgent state updates — mark as transitions
function SearchResults() {
  const [query, setQuery] = useState('');
  const [isPending, startTransition] = useTransition();
  const deferredQuery = useDeferredValue(query);

  return (
    <>
      <input
        value={query}
        onChange={(e) => {
          setQuery(e.target.value); // urgent: update input immediately
          startTransition(() => {
            // non-urgent: defer expensive filtering
          });
        }}
      />
      {isPending && <Spinner />}
      {/* deferredQuery triggers re-render without blocking input */}
      <ResultsList query={deferredQuery} />
    </>
  );
}

3. Virtualize Long Lists

// Use TanStack Virtual for large lists (1000+ items)
import { useVirtualizer } from '@tanstack/react-virtual';

function VirtualList({ items }: { items: Item[] }) {
  const parentRef = useRef<HTMLDivElement>(null);
  const virtualizer = useVirtualizer({
    count: items.length,
    getScrollElement: () => parentRef.current,
    estimateSize: () => 60, // estimated row height in px
    overscan: 5,
  });

  return (
    <div ref={parentRef} style={{ height: '600px', overflow: 'auto' }}>
      <div style={{ height: virtualizer.getTotalSize() }}>
        {virtualizer.getVirtualItems().map((vItem) => (
          <div
            key={vItem.key}
            style={{ position: 'absolute', top: vItem.start, height: vItem.size }}
          >
            <ItemRow item={items[vItem.index]} />
          </div>
        ))}
      </div>
    </div>
  );
}

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CLS Optimization Strategies

/* 1. Reserve space for images with aspect-ratio */
img, video {
  width: 100%;
  height: auto;
  aspect-ratio: 16 / 9; /* prevents layout shift before load */
}

/* 2. Reserve space for dynamic content */
.ad-container {
  min-height: 250px; /* reserve ad slot height */
}

/* 3. Use font-display: optional or swap */
@font-face {
  font-family: 'Inter';
  src: url('/fonts/inter.woff2') format('woff2');
  font-display: optional; /* no invisible text, no layout shift */
}

// 4. Avoid inserting content above existing content
// BAD: inserting notification banner at top pushes content down
setShowBanner(true); // causes CLS

// GOOD: use position:fixed or reserve space at the top
<div className="h-12">{showBanner && <Banner />}</div>

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Bundle Optimization

Next.js Bundle Analysis

# Analyze bundle composition
ANALYZE=true npm run build
# Or use @next/bundle-analyzer

Code Splitting Patterns

import dynamic from 'next/dynamic';

// Dynamic import — code splits automatically
const HeavyChart = dynamic(() => import('@/components/HeavyChart'), {
  loading: () => <ChartSkeleton />,
  ssr: false, // disable SSR for client-only components
});

// Lazy load with React.lazy (non-Next.js)
const Modal = React.lazy(() => import('./Modal'));

Tree-shaking Checklist

• Use named imports: import { debounce } from 'lodash-es' (not import _ from 'lodash')
• Set "sideEffects": false in package.json for libraries.
• Avoid import * from large libraries.
• Replace moment.js with date-fns or dayjs.
• Use sharp for server-side image processing.

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React Performance Patterns

// memo — skip re-render if props didn't change
const ExpensiveList = memo(({ items }: { items: Item[] }) => (
  <ul>{items.map((i) => <li key={i.id}>{i.name}</li>)}</ul>
));

// useCallback — stable function reference for memo children
const handleDelete = useCallback((id: string) => {
  setItems((prev) => prev.filter((i) => i.id !== id));
}, []); // empty deps = stable reference

// useMemo — memoize expensive calculations
const sortedItems = useMemo(
  () => [...items].sort((a, b) => a.name.localeCompare(b.name)),
  [items]
);

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Performance Monitoring Setup

// report-vitals.ts — send Core Web Vitals to analytics
import { onCLS, onINP, onLCP, onFCP, onTTFB } from 'web-vitals';

function sendToAnalytics({ name, value, id, navigationType }: Metric) {
  // Send to PostHog, Google Analytics, or your own endpoint
  fetch('/api/analytics/vitals', {
    method: 'POST',
    body: JSON.stringify({ name, value: Math.round(value), id, navigationType }),
    keepalive: true, // ensures request completes even if page unloads
  });
}

onCLS(sendToAnalytics);
onINP(sendToAnalytics);
onLCP(sendToAnalytics);
onFCP(sendToAnalytics);
onTTFB(sendToAnalytics);

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Lighthouse Score Improvement Checklist

• LCP image has priority or fetchpriority="high" + preload link.
• All non-LCP images use loading="lazy".
• Fonts loaded with font-display: optional or preloaded with rel="preload".
• No render-blocking <script> tags without defer or async.
• Bundle size < 200KB gzipped for initial JS.
• Images served in WebP/AVIF format with srcset.
• Server response time (TTFB) < 600ms — use CDN edge caching.
• Long tasks (> 50ms) are broken up or deferred.
• Virtual lists used for > 100 items.
• Unused CSS removed (PurgeCSS / Tailwind purge).
• HTTP/2 or HTTP/3 enabled on server.
• Resource hints: dns-prefetch, preconnect for third-party origins.

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<a name="bahasa-indonesia"></a>

Bahasa Indonesia

Integrasi Orkestrasi

Terhubung dan mengorkestrasi skill domain yang relevan seperti brainstorming, zero-to-prod-orchestrator, dan session-memory-manager untuk memastikan eksekusi yang kohesif.

Deskripsi

Panduan ahli untuk mengukur, mendiagnosis, dan mengoptimalkan performa aplikasi web dengan fokus pada Core Web Vitals (LCP, INP, CLS), optimasi JavaScript bundle, strategi loading gambar dan font, performa rendering, dan mencapai skor Lighthouse yang tinggi.

Kondisi Pemicu

• Meningkatkan skor Lighthouse di bawah 90.
• Mendiagnosis metrik LCP, INP, atau CLS yang buruk.
• Mengurangi ukuran JavaScript bundle atau menghilangkan resource yang memblokir render.
• Mengoptimalkan loading gambar, lazy loading, atau pengiriman font.
• Mengimplementasikan pola performa React (memo, useDeferredValue, Suspense).
• Menyiapkan Real User Monitoring (RUM) untuk Core Web Vitals.

Target Core Web Vitals

Metrik	Baik	Perlu Perbaikan	Buruk
LCP	≤ 2,5 detik	2,5–4,0 detik	> 4,0 detik
INP	≤ 200ms	200–500ms	> 500ms
CLS	≤ 0,1	0,1–0,25	> 0,25

Ringkasan Strategi Optimasi

• LCP: Preload hero image, gunakan priority di Next.js Image, optimalkan TTFB dengan CDN.
• INP: Pecah long task dengan scheduler.yield(), gunakan useTransition React 19, virtualisasi list panjang.
• CLS: Reservasi ruang untuk gambar dengan aspect-ratio, hindari penyisipan konten di atas konten yang sudah ada.
• Bundle: Analisis bundle, gunakan dynamic import, tree-shaking, dan named imports.
• Monitoring: Implementasikan web-vitals library untuk melaporkan metrik ke analytics secara real-time.