Optimize

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

Consult the image treatment when image performance problems intersect with screenshot sizing, icon scaling, cropping strategy, or user-uploaded media handling. Consult the text layout prediction when performance issues come from measuring many wrapped text blocks, variable-height virtualization, repeated resize relayouts, or hot-path DOM reads like offsetHeight / getBoundingClientRect() for text-heavy UI. Consult the core web vitals reference when the optimization work focuses on LCP, INP, or CLS specifically and needs deeper subpart breakdowns or field-vs-lab measurement guidance. When a project needs virtualization for very long lists and the stack is still open, prefer TanStack Virtual as the default headless virtualization layer across the supported React, Vue, Angular, Solid, and Svelte ecosystems. If the project already uses another virtualization layer, preserve that first.

MANDATORY PREPARATION

Users start this workflow with /optimize. Once this skill is active, load $frontend-design — it contains design principles, anti-patterns, and the Context Gathering Protocol. Follow that protocol before proceeding — if no design context exists yet, you MUST load $setup first. Additionally gather: the target devices, performance constraints, and which user interactions feel slow.

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

• Interaction latency: Response time for filters, search, save, open-panel, autocomplete, and other repeated actions
• 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? Narrow-layout users? Slow connections?)

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

Optimization Strategy

Create systematic improvement plan:

Doherty Threshold for Interaction Speed

Routine interactions should preserve flow. Aim for users to see acknowledgment immediately and, when possible, feel the interaction resolve within roughly 400ms.

When work will exceed that window:

• show instant feedback within ~100ms (pressed, active, loading, optimistic state)
• stream or reveal partial content early instead of waiting for everything
• prefetch likely next screens or data for common paths
• prefer skeletons, optimistic UI, and progressive rendering over blank waits and generic spinners

Don't rely on a spinner as the main experience for high-frequency actions if the interface could instead acknowledge intent and keep momentum alive.

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"
/>

Modern image format strategy (AVIF and WebP):

AVIF typically delivers 30-50% smaller files than WebP at equivalent visual quality. WebP has broader baseline support. Use both with fallback:

<picture>
  <source srcset="hero.avif" type="image/avif">
  <source srcset="hero.webp" type="image/webp">
  <img src="hero.jpg" alt="Hero image" loading="lazy">
</picture>

Practical rules:

• generate AVIF for hero images, large backgrounds, and photo-heavy galleries where the size savings matter most
• keep WebP as the fallback for broader browser coverage
• do not serve AVIF for tiny icons or simple graphics where encoding overhead outweighs savings
• use an image CDN that handles format negotiation automatically when possible
• test AVIF decode speed on low-end devices; the format is smaller but can be slower to decode than WebP

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 */
}

Font subsetting strategy:

Subsetting removes unused glyphs and dramatically reduces file size. A full Latin font may be 200KB; the same font subsetted to Basic Latin can be 20-30KB.

Approaches:

• Static subsetting: generate separate font files per language or script (Latin, Cyrillic, CJK) and serve only what the page needs via unicode-range
• Dynamic subsetting: use a service like Google Fonts or a self-hosted subsetter that generates slices on demand
• Critical glyph subsetting: for above-the-fold hero text, subset to only the characters used in the headline and preload that file

/* Latin-only subset for body text */
@font-face {
  font-family: 'CustomFont';
  src: url('/fonts/custom-latin.woff2') format('woff2');
  font-display: swap;
  unicode-range: U+0020-007F, U+00A0-00FF;
}

/* Extended subset for other scripts, loaded only when needed */
@font-face {
  font-family: 'CustomFont';
  src: url('/fonts/custom-cyrillic.woff2') format('woff2');
  font-display: swap;
  unicode-range: U+0400-04FF;
}

Tools for subsetting:

• pyftsubset (fonttools) for precise manual subsetting
• subfont for automatic subsetting at build time
• glyphhanger for extracting only the glyphs used in your HTML/CSS

Critical CSS extraction:

Extract the CSS needed for above-the-fold content and inline it in the HTML <head>. Load the remaining CSS asynchronously.

Why it matters:

• render-blocking CSS is often the biggest contributor to slow First Contentful Paint
• inlining critical CSS eliminates the network request for the initial view
• non-critical CSS can load after the page is interactive

How to implement:

• use critical (npm package) or Penthouse to extract above-the-fold CSS automatically
• or identify critical CSS manually: header styles, hero layout, initial typography, and above-the-fold component styles
• inline the critical CSS in a <style> tag in the <head>
• load the full stylesheet asynchronously:

<head>
  <style>
    /* Critical CSS inlined here */
  </style>
  <link rel="preload" href="/styles.css" as="style"
  <noscript><link rel="stylesheet" href="/styles.css"></noscript>
</head>

Caveats:

• do not inline more than ~14KB of CSS (gzippped) to stay within single TCP round-trip budgets
• critical CSS must be kept in sync with the actual above-the-fold content; regenerate it when layouts change
• for SPAs, consider route-level critical CSS rather than page-level

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

Container queries for component-level responsiveness:

Container queries let components adapt to their own container size rather than the viewport. This reduces the need for page-wide breakpoint overrides and makes components more reusable.

When to use:

• a component appears in multiple contexts (sidebar, main column, modal)
• the component layout should depend on the space it has, not the device width
• you want to reduce breakpoint proliferation at the page level

Consult the container queries reference for syntax, practical patterns, and browser support.

Performance note:

• container queries add minimal runtime cost; they run on the compositor like media queries
• prefer container-type: inline-size over size unless you genuinely need both dimensions
• avoid creating containment contexts on every element; only where the component needs to query its container

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 — prefer TanStack Virtual when the architecture is still open; preserve the existing virtualization stack first if the project already has one
• For text-heavy variable-height lists or cards, prefer predictive text measurement over repeated live DOM height reads when wrapping is the real bottleneck

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 constrained-browser performance (often slower hardware, tighter CPU budgets, 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-power touch-capable hardware and mainstream laptop/desktop browsers
• 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.