optimize

name	optimize
description	Diagnoses and fixes UI performance across loading speed, rendering, animations, images, and bundle size. Use when the user mentions slow, laggy, janky, performance, bundle size, load time, or wants a faster, smoother experience.
user-invocable	true
argument-hint	[target]

Identify and fix performance issues to create faster, smoother user experiences.

Assess Performance Issues

Understand current performance and identify problems:

1. Measure current state:

   • Core Web Vitals: LCP, FID/INP, CLS scores
   • Load time: Time to interactive, first contentful paint
   • Bundle size: JavaScript, CSS, image sizes
   • Runtime performance: Frame rate, memory usage, CPU usage
   • Network: Request count, payload sizes, waterfall

2. Identify bottlenecks:

   • What's slow? (Initial load? Interactions? Animations?)
   • What's causing it? (Large images? Expensive JavaScript? Layout thrashing?)
   • How bad is it? (Perceivable? Annoying? Blocking?)
   • Who's affected? (All users? Mobile only? Slow connections?)

CRITICAL: Measure before and after. Premature optimization wastes time. Optimize what actually matters.

Optimization Strategy

Create systematic improvement plan:

Loading Performance

Optimize Images:

• Use modern formats (WebP, AVIF)
• Proper sizing (don't load 3000px image for 300px display)
• Lazy loading for below-fold images
• Responsive images (srcset, picture element)
• Compress images (80-85% quality is usually imperceptible)
• Use CDN for faster delivery

<img
	src="hero.webp"
	srcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w"
	sizes="(max-width: 400px) 400px, (max-width: 800px) 800px, 1200px"
	loading="lazy"
	alt="Hero image"
/>

Reduce JavaScript Bundle:

• Code splitting (route-based, component-based)
• Tree shaking (remove unused code)
• Remove unused dependencies
• Lazy load non-critical code
• Use dynamic imports for large components

// Lazy load heavy component
const HeavyChart = lazy(() => import("./HeavyChart"));

Optimize CSS:

• Remove unused CSS
• Critical CSS inline, rest async
• Minimize CSS files
• Use CSS containment for independent regions

Optimize Fonts:

• Use font-display: swap or optional
• Subset fonts (only characters you need)
• Preload critical fonts
• Use system fonts when appropriate
• Limit font weights loaded

@font-face {
	font-family: "CustomFont";
	src: url("/fonts/custom.woff2") format("woff2");
	font-display: swap; /* Show fallback immediately */
	unicode-range: U+0020-007F; /* Basic Latin only */
}

Optimize Loading Strategy:

• Critical resources first (async/defer non-critical)
• Preload critical assets
• Prefetch likely next pages
• Service worker for offline/caching
• HTTP/2 or HTTP/3 for multiplexing

Rendering Performance

Avoid Layout Thrashing:

// ❌ Bad: Alternating reads and writes (causes reflows)
elements.forEach((el) => {
	const height = el.offsetHeight; // Read (forces layout)
	el.style.height = height * 2; // Write
});

// ✅ Good: Batch reads, then batch writes
const heights = elements.map((el) => el.offsetHeight); // All reads
elements.forEach((el, i) => {
	el.style.height = heights[i] * 2; // All writes
});

Optimize Rendering:

• Use CSS contain property for independent regions
• Minimize DOM depth (flatter is faster)
• Reduce DOM size (fewer elements)
• Use content-visibility: auto for long lists
• Virtual scrolling for very long lists (react-window, react-virtualized)

Reduce Paint & Composite:

• Use transform and opacity for animations (GPU-accelerated)
• Avoid animating layout properties (width, height, top, left)
• Use will-change sparingly for known expensive operations
• Minimize paint areas (smaller is faster)

Animation Performance

GPU Acceleration:

/* ✅ GPU-accelerated (fast) */
.animated {
	transform: translateX(100px);
	opacity: 0.5;
}

/* ❌ CPU-bound (slow) */
.animated {
	left: 100px;
	width: 300px;
}

Smooth 60fps:

• Target 16ms per frame (60fps)
• Use requestAnimationFrame for JS animations
• Debounce/throttle scroll handlers
• Use CSS animations when possible
• Avoid long-running JavaScript during animations

Intersection Observer:

// Efficiently detect when elements enter viewport
const observer = new IntersectionObserver((entries) => {
	entries.forEach((entry) => {
		if (entry.isIntersecting) {
			// Element is visible, lazy load or animate
		}
	});
});

React/Framework Optimization

React-specific:

• Use memo() for expensive components
• useMemo() and useCallback() for expensive computations
• Virtualize long lists
• Code split routes
• Avoid inline function creation in render
• Use React DevTools Profiler

Framework-agnostic:

• Minimize re-renders
• Debounce expensive operations
• Memoize computed values
• Lazy load routes and components

Network Optimization

Reduce Requests:

• Combine small files
• Use SVG sprites for icons
• Inline small critical assets
• Remove unused third-party scripts

Optimize APIs:

• Use pagination (don't load everything)
• GraphQL to request only needed fields
• Response compression (gzip, brotli)
• HTTP caching headers
• CDN for static assets

Optimize for Slow Connections:

• Adaptive loading based on connection (navigator.connection)
• Optimistic UI updates
• Request prioritization
• Progressive enhancement

Core Web Vitals Optimization

Largest Contentful Paint (LCP < 2.5s)

• Optimize hero images
• Inline critical CSS
• Preload key resources
• Use CDN
• Server-side rendering

First Input Delay (FID < 100ms) / INP (< 200ms)

• Break up long tasks
• Defer non-critical JavaScript
• Use web workers for heavy computation
• Reduce JavaScript execution time

Cumulative Layout Shift (CLS < 0.1)

• Set dimensions on images and videos
• Don't inject content above existing content
• Use aspect-ratio CSS property
• Reserve space for ads/embeds
• Avoid animations that cause layout shifts

/* Reserve space for image */
.image-container {
	aspect-ratio: 16 / 9;
}

Performance Monitoring

Tools to use:

• Chrome DevTools (Lighthouse, Performance panel)
• WebPageTest
• Core Web Vitals (Chrome UX Report)
• Bundle analyzers (webpack-bundle-analyzer)
• Performance monitoring (Sentry, DataDog, New Relic)

Key metrics:

• LCP, FID/INP, CLS (Core Web Vitals)
• Time to Interactive (TTI)
• First Contentful Paint (FCP)
• Total Blocking Time (TBT)
• Bundle size
• Request count

IMPORTANT: Measure on real devices with real network conditions. Desktop Chrome with fast connection isn't representative.

NEVER:

• Optimize without measuring (premature optimization)
• Sacrifice accessibility for performance
• Break functionality while optimizing
• Use will-change everywhere (creates new layers, uses memory)
• Lazy load above-fold content
• Optimize micro-optimizations while ignoring major issues (optimize the biggest bottleneck first)
• Forget about mobile performance (often slower devices, slower connections)

Verify Improvements

Test that optimizations worked:

• Before/after metrics: Compare Lighthouse scores
• Real user monitoring: Track improvements for real users
• Different devices: Test on low-end Android, not just flagship iPhone
• Slow connections: Throttle to 3G, test experience
• No regressions: Ensure functionality still works
• User perception: Does it feel faster?

Remember: Performance is a feature. Fast experiences feel more responsive, more polished, more professional. Optimize systematically, measure ruthlessly, and prioritize user-perceived performance.