web-performance

Performance optimization

Evidence-led performance optimization using real-user signals for prioritization and browser traces for diagnosis. Focuses on loading speed, runtime responsiveness, and resource delivery.

How it works

1. If a page can run, read the measurement workflow and establish a field-plus-lab baseline before editing.
2. Prioritize poor real-user Core Web Vitals. Use a DevTools performance trace and its focused insights to find the cause.
3. Inspect and change only the code or assets connected to measured bottlenecks.
4. Re-run equivalent lab measurements and report before/after values, conditions, and uncertainty. Field verification remains pending until enough new user data arrives.

When no runnable page exists, perform static inspection but call findings hypotheses, not measured regressions. Include the command or browser workflow that can verify each high-impact hypothesis.

Prefer a browser tool that records a performance trace and exposes focused insights. With Chrome DevTools MCP, use performance_start_trace and performance_analyze_insight; do not route performance through lighthouse_audit, which covers non-performance Lighthouse categories.

Starting performance budget

Budgets must reflect the product's target devices, networks, page types, and user journeys. The values below are initial guardrails for a typical content or commerce page, not universal pass/fail criteria. Preserve an existing project budget when one is already defined.

Resource	Budget	Rationale
Total page weight	< 1.5 MB	Bounds transfer time and data cost on constrained target networks; calibrate with representative pages
JavaScript (compressed)	< 300 KB	Protect parse and execution cost
CSS (compressed)	< 100 KB	Limit render-blocking work
Images (above-fold)	< 500 KB	Protect likely LCP resources
Fonts	< 100 KB	Limit critical font transfer
Third-party	< 200 KB	Bound code outside product control

Critical rendering path

Server response

• TTFB < 800ms. Time to First Byte should be fast. Use CDN, caching, and efficient backends.
• Enable compression. Gzip or Brotli for text assets. Brotli preferred (15-20% smaller).
• HTTP/2 or HTTP/3. Multiplexing reduces connection overhead.
• Edge caching. Cache HTML at CDN edge when possible.
• Consider Early Hints (HTTP 103) for measured document latency. If a trace shows slow HTML generation and stable critical subresources, send an interim 103 with Link headers before the normal final response from the same request. Use HTTP/2 or later. A CDN may synthesize the 103 from Link headers on an earlier 200, or the origin/edge handler can emit it directly. Unsupported clients continue to the final response, but confirm current browser and infrastructure support. Limit hints to proven critical preloads or preconnects: inaccurate hints waste bandwidth. Cloudflare reported a 20–30% LCP improvement in an artificial, image-heavy test; treat that as a vendor case study, not an expected saving, and measure your result. See MDN's 103 implementation example and the Cloudflare study.

Resource loading

Preconnect to required origins:

<link rel="preconnect" href="https://fonts.googleapis.com" />
<link rel="preconnect" href="https://cdn.example.com" crossorigin />

Preload critical resources:

Preload only resources whose late discovery is visible in the trace. Each preload competes for bandwidth and an unnecessary high-priority request can delay LCP.

<!-- LCP image -->
<link rel="preload" href="/hero.webp" as="image" fetchpriority="high" />

<!-- Critical font -->
<link
  rel="preload"
  href="/font.woff2"
  as="font"
  type="font/woff2"
  crossorigin
/>

Prerender likely-next navigations with the Speculation Rules API:

<script type="speculationrules">
  {
    "prerender": [
      {
        "where": { "href_matches": "/*" },
        "eagerness": "moderate"
      }
    ]
  }
</script>

moderate waits for a stronger intent signal than eager modes. Measure prediction hit rate, transferred bytes, and server cost; a wrong prerender is roughly an unused navigation. See core-web-vitals → LCP for the tradeoffs and the prerenderingchange gating needed for analytics.

Defer non-critical CSS:

<!-- Critical CSS inlined -->
<style>
  /* Above-fold styles */
</style>

<!-- Non-critical CSS -->
<link
  rel="preload"
  href="/styles.css"
  as="style"
  onload="this.onload=null;this.rel='stylesheet'"
/>
<noscript><link rel="stylesheet" href="/styles.css" /></noscript>

JavaScript optimization

Defer non-essential scripts:

<!-- Parser-blocking (avoid) -->
<script src="/critical.js"></script>

<!-- Deferred (preferred) -->
<script defer src="/app.js"></script>

<!-- Async (for independent scripts) -->
<script async src="/analytics.js"></script>

<!-- Module (deferred by default) -->
<script type="module" src="/app.mjs"></script>

Code splitting patterns:

// Route-based splitting
const Dashboard = lazy(() => import("./Dashboard"));

// Component-based splitting
const HeavyChart = lazy(() => import("./HeavyChart"));

// Feature-based splitting
if (user.isPremium) {
  const PremiumFeatures = await import("./PremiumFeatures");
}

Tree shaking best practices:

// ❌ Imports entire library
import _ from "lodash";
_.debounce(fn, 300);

// ✅ Imports only what's needed
import debounce from "lodash/debounce";
debounce(fn, 300);

Image optimization

Format selection

Format	Use case	Browser support
AVIF	Photos, best compression	92%+
WebP	Photos, good fallback	97%+
PNG	Graphics with transparency	Universal
SVG	Icons, logos, illustrations	Universal

Responsive images

<picture>
  <!-- AVIF for modern browsers -->
  <source
    type="image/avif"
    srcset="hero-400.avif 400w, hero-800.avif 800w, hero-1200.avif 1200w"
    sizes="(max-width: 600px) 100vw, 50vw"
  />

  <!-- WebP fallback -->
  <source
    type="image/webp"
    srcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w"
    sizes="(max-width: 600px) 100vw, 50vw"
  />

  <!-- JPEG fallback -->
  <img
    src="hero-800.jpg"
    srcset="hero-400.jpg 400w, hero-800.jpg 800w, hero-1200.jpg 1200w"
    sizes="(max-width: 600px) 100vw, 50vw"
    width="1200"
    height="600"
    alt="Hero image"
    loading="lazy"
    decoding="async"
  />
</picture>

LCP image priority

<!-- Above-fold LCP image: eager loading, high priority -->
<img
  src="hero.webp"
  fetchpriority="high"
  loading="eager"
  decoding="sync"
  alt="Hero"
/>

<!-- Below-fold images: lazy loading -->
<img src="product.webp" loading="lazy" decoding="async" alt="Product" />

Font optimization

Loading strategy

/* System font stack as fallback */
body {
  font-family:
    "Custom Font",
    -apple-system,
    BlinkMacSystemFont,
    "Segoe UI",
    Roboto,
    sans-serif;
}

/* Prevent invisible text */
@font-face {
  font-family: "Custom Font";
  src: url("/fonts/custom.woff2") format("woff2");
  font-display: swap; /* or optional for non-critical */
  font-weight: 400;
  font-style: normal;
  unicode-range: U+0000-00FF; /* Subset to Latin */
}

Preloading critical fonts

<link
  rel="preload"
  href="/fonts/heading.woff2"
  as="font"
  type="font/woff2"
  crossorigin
/>

Variable fonts

/* One file instead of multiple weights */
@font-face {
  font-family: "Inter";
  src: url("/fonts/Inter-Variable.woff2") format("woff2-variations");
  font-weight: 100 900;
  font-display: swap;
}

Caching strategy

Cache-Control headers

# HTML (short or no cache)
Cache-Control: no-cache, must-revalidate

# Static assets with hash (immutable)
Cache-Control: public, max-age=31536000, immutable

# Static assets without hash
Cache-Control: public, max-age=86400, stale-while-revalidate=604800

# API responses
Cache-Control: private, max-age=0, must-revalidate

Service worker caching

// Cache-first for static assets
self.addEventListener("fetch", (event) => {
  if (
    event.request.destination === "image" ||
    event.request.destination === "style" ||
    event.request.destination === "script"
  ) {
    event.respondWith(
      caches.match(event.request).then((cached) => {
        return (
          cached ||
          fetch(event.request).then((response) => {
            const clone = response.clone();
            caches
              .open("static-v1")
              .then((cache) => cache.put(event.request, clone));
            return response;
          })
        );
      }),
    );
  }
});

Runtime performance

Avoid layout thrashing

// ❌ Forces multiple reflows
elements.forEach((el) => {
  const height = el.offsetHeight; // Read
  el.style.height = height + 10 + "px"; // Write
});

// ✅ Batch reads, then batch writes
const heights = elements.map((el) => el.offsetHeight); // All reads
elements.forEach((el, i) => {
  el.style.height = heights[i] + 10 + "px"; // All writes
});

Debounce expensive operations

function debounce(fn, delay) {
  let timeout;
  return (...args) => {
    clearTimeout(timeout);
    timeout = setTimeout(() => fn(...args), delay);
  };
}

// Debounce scroll/resize handlers
window.addEventListener("scroll", debounce(handleScroll, 100));

Use requestAnimationFrame

// ❌ May cause jank
setInterval(animate, 16);

// ✅ Synced with display refresh
function animate() {
  // Animation logic
  requestAnimationFrame(animate);
}
requestAnimationFrame(animate);

Virtualize long lists

// For lists > 100 items, render only visible items
// Use libraries like react-window, vue-virtual-scroller, or native CSS:
.virtual-list {
  content-visibility: auto;
  contain-intrinsic-size: 0 50px; /* Estimated item height */
}

Smooth navigations with View Transitions

The View Transitions API lets the browser cross-fade (or custom-animate) between two DOM states using a single GPU-composited snapshot — no double-render, no layout thrash, and the snapshot doesn't count toward CLS.

Same-document (SPA-style) — Baseline 2026:

// Wrap the DOM mutation that swaps the view
function navigate(newView) {
  if (!document.startViewTransition) return swapDOM(newView);
  document.startViewTransition(() => swapDOM(newView));
}

Cross-document (MPA-style) — Chromium-stable, progressive enhancement elsewhere:

/* On both source and destination pages */
@view-transition {
  navigation: auto;
}

That's the entire integration — same-origin navigations now fade automatically. To opt specific elements into shared-element transitions (e.g. a thumbnail expanding into a hero), give them a matching view-transition-name:

.product-thumb[data-id="42"],
.product-hero {
  view-transition-name: product-42;
}

Pair this with Speculation Rules (above) for instant + animated navigations.

Third-party scripts

Load strategies

// ❌ Blocks main thread
<script src="https://analytics.example.com/script.js"></script>

// ✅ Async loading
<script async src="https://analytics.example.com/script.js"></script>

// ✅ Delay until interaction
<script>
document.addEventListener('DOMContentLoaded', () => {
  const observer = new IntersectionObserver((entries) => {
    if (entries[0].isIntersecting) {
      const script = document.createElement('script');
      script.src = 'https://widget.example.com/embed.js';
      document.body.appendChild(script);
      observer.disconnect();
    }
  });
  observer.observe(document.querySelector('#widget-container'));
});
</script>

Facade pattern

<!-- Show static placeholder until interaction -->
<div class="youtube-facade" data-video-id="abc123" onclick="loadYouTube(this)">
  <img src="/thumbnails/abc123.jpg" alt="Video title" />
  <button aria-label="Play video">▶</button>
</div>

Measurement

Use the measurement workflow whenever a URL is runnable. It defines Chrome DevTools MCP routing, CrUX and fallback sources, repeatable lab conditions, and a compact evidence format.

Metric	Kind	Interpretation
LCP, INP, CLS at p75	Field	User-outcome Core Web Vitals; use for pass/fail prioritization
LCP, CLS in a trace	Lab	Reproducible diagnostic values for one navigation
TBT	Lab	Main-thread blocking diagnostic and a rough INP proxy, not field INP
FCP, Speed Index	Lab	Loading diagnostics, not Core Web Vitals

Raw PerformanceObserver snippets are useful for the current browser session but are not real-user data by themselves. When the user wants production telemetry, read the first-party RUM reference and prefer web-vitals over a hand-rolled metric implementation.

References

For Core Web Vitals specific optimizations, see Core Web Vitals.