roedyrustam Web Performance & Core Web Vitals Expert
English
Orchestration & Integration
Connects and orchestrates with relevant domain skills like brainstorming, zero-to-prod-orchestrator, and project-context-mapper 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).
Core Web Vitals Targets (2024+)
| 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 |
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
/>
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}
=> {
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>
);
}
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>
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'(notimport _ from 'lodash') - Set
"sideEffects": falseinpackage.jsonfor libraries. - Avoid
import *from large libraries. - Replace
moment.jswithdate-fnsordayjs. - Use
sharpfor server-side image processing.
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]
);
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);
Lighthouse Score Improvement Checklist
- LCP image has
priorityorfetchpriority="high"+ preload link. - All non-LCP images use
loading="lazy". - Fonts loaded with
font-display: optionalor preloaded withrel="preload". - No render-blocking
<script>tags withoutdeferorasync. - 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,preconnectfor third-party origins.
Bahasa Indonesia
Integrasi Orkestrasi
Terhubung dan mengorkestrasi skill domain yang relevan seperti brainstorming, zero-to-prod-orchestrator, dan project-context-mapper 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
prioritydi Next.js Image, optimalkan TTFB dengan CDN. - INP: Pecah long task dengan
scheduler.yield(), gunakanuseTransitionReact 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-vitalslibrary untuk melaporkan metrik ke analytics secara real-time.