Web Performance Engine

Complete web performance optimization system. Audit, diagnose, fix, and monitor — no external tools required (but integrates with Lighthouse, WebPageTest, Chrome DevTools when available).

Phase 1: Performance Audit

Quick Health Check

Run these checks in order. Stop when you find the bottleneck tier.

Tier 1 — Critical (blocks rendering):

Time to First Byte (TTFB) > 800ms → server problem
First Contentful Paint (FCP) > 1.8s → render-blocking resources
Largest Contentful Paint (LCP) > 2.5s → hero element problem
Total Blocking Time (TBT) > 200ms → JavaScript problem
Cumulative Layout Shift (CLS) > 0.1 → layout instability
Interaction to Next Paint (INP) > 200ms → event handler problem

Tier 2 — Important (affects experience):

Tier 3 — Polish (competitive edge):

Speed Index > 3.4s
Time to Interactive > 3.8s
Font loading causes flash
Third-party scripts > 30% of JS

Audit Brief Template

audit:
  url: ""
  device: "mobile"  # mobile | desktop | both
  connection: "4G"  # 3G | 4G | fiber
  region: ""        # closest to target users
  
scores:
  performance: null  # 0-100
  fcp_ms: null
  lcp_ms: null
  tbt_ms: null
  cls: null
  inp_ms: null
  ttfb_ms: null
  
page_weight:
  total_kb: null
  html_kb: null
  css_kb: null
  js_kb: null
  images_kb: null
  fonts_kb: null
  other_kb: null
  
requests:
  total: null
  by_type: {}
  third_party_count: null
  third_party_kb: null

Getting Metrics Without Tools

If no Lighthouse/DevTools available, use web-based tools:

web_fetch "https://pagespeed.web.dev/analysis?url={encoded_url}" — Google's free tool
web_search "webpagetest {url}" — find cached results
web_search "site:{domain} core web vitals" — find CrUX data
Check <head> for obvious issues: render-blocking CSS/JS, missing preloads, no meta viewport

Phase 2: Diagnosis — The Performance Waterfall

Critical Rendering Path Analysis

DNS → TCP → TLS → TTFB → HTML Parse → CSSOM → Render Tree → FCP → LCP
                                ↓
                         JS Download → Parse → Execute → INP

Bottleneck Decision Tree:

High TTFB (>800ms)?
├─ YES → Phase 3A: Server optimization
└─ NO → High FCP (>1.8s)?
    ├─ YES → Phase 3B: Render-blocking resources
    └─ NO → High LCP (>2.5s)?
        ├─ YES → Phase 3C: Hero element optimization
        └─ NO → High TBT (>200ms)?
            ├─ YES → Phase 3D: JavaScript optimization
            └─ NO → High CLS (>0.1)?
                ├─ YES → Phase 3E: Layout stability
                └─ NO → High INP (>200ms)?
                    ├─ YES → Phase 3F: Interaction optimization
                    └─ NO → ✅ Performance is good!

Resource Impact Scoring

Rate each resource by impact:

Factor	Weight	Score 1	Score 3	Score 5
Size (KB)	3x	<10	10-100	>100
Render-blocking	5x	No	Partial	Full
Above-fold impact	4x	None	Indirect	Direct
Cacheable	2x	Long cache	Short cache	No cache
Compressible	2x	Already done	Possible	Not compressed

Priority = Sum(Factor × Weight). Fix highest scores first.

Phase 3: Fix Playbooks

3A: Server Optimization (TTFB)

Quick wins:

# CDN: If no CDN, this is #1 priority
# Check: curl -sI {url} | grep -i 'x-cache\|cf-cache\|x-cdn'

# Compression: Must have brotli or gzip
# Check: curl -sI -H "Accept-Encoding: br,gzip" {url} | grep -i content-encoding

# HTTP/2 or HTTP/3
# Check: curl -sI --http2 {url} | head -1

Server-side checklist:

CDN in front (Cloudflare, Fastly, CloudFront)
Brotli compression enabled (20-30% smaller than gzip)
HTTP/2 minimum, HTTP/3 if possible
Server-side caching (Redis, Varnish)
Database query optimization (<50ms per query)
Connection pooling enabled
Edge computing for dynamic content (Workers, Lambda@Edge)

Cache headers template:

# Static assets (CSS, JS, images, fonts)
Cache-Control: public, max-age=31536000, immutable

# HTML pages
Cache-Control: public, max-age=0, must-revalidate

# API responses
Cache-Control: private, max-age=60, stale-while-revalidate=300

3B: Render-Blocking Resources (FCP)

CSS optimization:

<!-- BEFORE: Render-blocking -->
<link rel="stylesheet" href="styles.css">

<!-- AFTER: Critical CSS inline + async load -->
<style>/* Critical above-fold CSS here (< 14KB) */</style>
<link rel="preload" href="styles.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
<noscript><link rel="stylesheet" href="styles.css"></noscript>

Rules:

Inline critical CSS (above-fold styles, < 14KB)
Defer non-critical CSS
Remove unused CSS (typical savings: 60-90%)
Combine media queries
Avoid @import (creates sequential loading)

JavaScript optimization:

<!-- BEFORE: Render-blocking -->
<script src="app.js"></script>

<!-- AFTER: Non-blocking -->
<script src="app.js" defer></script>

<!-- OR: Independent scripts -->
<script src="analytics.js" async></script>

Rules:

defer for app scripts (maintains order, runs after parse)
async for independent scripts (analytics, ads)
Never put <script> in <head> without defer/async
Inline small scripts (< 1KB)

3C: Hero Element Optimization (LCP)

LCP element types and fixes:

LCP Element	Fix
`<img>`	Preload + responsive + modern format
`<video>` poster	Preload poster image
CSS `background-image`	Preload + inline critical CSS
Text block	Preload font + font-display: optional

Image optimization checklist:

<!-- Optimal hero image -->
<link rel="preload" as="image" href="hero.webp" 
      imagesrcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w"
      imagesizes="100vw">

<img src="hero.webp" 
     srcset="hero-400.webp 400w, hero-800.webp 800w, hero-1200.webp 1200w"
     sizes="100vw"
     width="1200" height="600"
     alt="Hero description"
     fetchpriority="high"
     decoding="async">

Image format decision:

Photo/complex image? → WebP (25-35% smaller than JPEG)
                     → AVIF (50% smaller, but slower encode)
Simple graphic/logo? → SVG (scalable, tiny)
                    → PNG only if transparency needed
Animation?          → WebM/MP4 video (not GIF — 90% smaller)

Image size targets:

Viewport	Max width	Target KB
Mobile	400px	< 50KB
Tablet	800px	< 100KB
Desktop	1200px	< 150KB
Hero/banner	1600px	< 200KB

3D: JavaScript Optimization (TBT)

Bundle analysis approach:

Check total JS size: web_fetch the page, count <script> tags
Identify large libraries (React, Lodash, Moment.js)
Check for duplicate code across bundles
Identify unused exports

Common JS bloat and replacements:

Library	Size	Alternative	Size
moment.js	67KB	date-fns	2-10KB
lodash (full)	71KB	lodash-es (tree-shake)	2-5KB
jQuery	87KB	vanilla JS	0KB
animate.css	80KB	CSS animations	1-2KB
chart.js	60KB	lightweight-charts	40KB

Code splitting rules:

Route-based splitting (each page loads its own JS)
Component-level splitting for heavy components (modals, editors, charts)
Dynamic import for below-fold features: const Chart = lazy(() => import('./Chart'))
Vendor chunk for stable dependencies (changes rarely = long cache)

Long task breaking:

// BEFORE: Blocks main thread 200ms+
function processLargeList(items) {
  items.forEach(item => heavyComputation(item));
}

// AFTER: Yields to main thread
async function processLargeList(items) {
  for (const item of items) {
    heavyComputation(item);
    // Yield every 50ms
    if (performance.now() - start > 50) {
      await scheduler.yield(); // or setTimeout(0)
      start = performance.now();
    }
  }
}

3E: Layout Stability (CLS)

Top CLS causes and fixes:

Cause	Fix
Images without dimensions	Always set `width` + `height`
Ads/embeds without space	Reserve space with `aspect-ratio` or `min-height`
Dynamic content injection	Use CSS `contain` or reserved space
Web fonts causing reflow	`font-display: optional` or `swap` with size-adjust
Late-loading CSS	Inline critical CSS

Anti-CLS patterns:

/* Reserve space for dynamic content */
.ad-slot { min-height: 250px; }
.embed-container { aspect-ratio: 16/9; }

/* Prevent font swap reflow */
@font-face {
  font-family: 'Brand';
  src: url('brand.woff2') format('woff2');
  font-display: optional; /* No swap = no shift */
  size-adjust: 105%; /* Match fallback metrics */
}

/* Contain layout shifts */
.dynamic-widget {
  contain: layout;
  min-height: 200px;
}

3F: Interaction Optimization (INP)

Event handler rules:

Keep handlers < 50ms
Debounce scroll/resize (100-150ms)
Use requestAnimationFrame for visual updates
Offload heavy computation to Web Workers
Use content-visibility: auto for off-screen content

Input responsiveness:

// BEFORE: Blocks during type
input.addEventListener('input', (e) => {
  expensiveFilter(e.target.value); // 100ms+ 
});

// AFTER: Debounced + visual feedback
input.addEventListener('input', (e) => {
  showSpinner(); // Instant visual feedback
  debounce(() => expensiveFilter(e.target.value), 150);
});

Phase 4: Resource Loading Strategy

Preload / Prefetch / Preconnect Decision

<!-- Preconnect: Third-party origins you'll need soon -->
<link rel="preconnect" href="https://fonts.googleapis.com">
<link rel="preconnect" href="https://cdn.example.com" crossorigin>

<!-- DNS-prefetch: Third-party origins you might need -->
<link rel="dns-prefetch" href="https://analytics.example.com">

<!-- Preload: Critical resources for THIS page -->
<link rel="preload" href="critical.css" as="style">
<link rel="preload" href="hero.webp" as="image">
<link rel="preload" href="brand.woff2" as="font" type="font/woff2" crossorigin>

<!-- Prefetch: Resources for NEXT page (low priority) -->
<link rel="prefetch" href="/next-page.js">

<!-- Modulepreload: ES modules -->
<link rel="modulepreload" href="app.mjs">

Rules:

Max 3-5 preloads per page (more = competing priorities)
Always preload: LCP image, critical font, above-fold CSS
Preconnect to known third-party origins (max 4-6)
Prefetch only on fast connections

Lazy Loading Strategy

Above fold (viewport):     fetchpriority="high", no lazy
Below fold (1-2 screens):  loading="lazy", decoding="async"
Way below fold:            Intersection Observer, load on demand
Off-screen widgets:        content-visibility: auto

Font Loading Optimization

/* Optimal font loading */
@font-face {
  font-family: 'Brand';
  src: url('brand.woff2') format('woff2');
  font-display: swap;
  unicode-range: U+0000-00FF; /* Latin only if applicable */
}

Font checklist:

WOFF2 format only (best compression)
Subset fonts (Latin, extended only if needed)
Max 2-3 font families
Max 4 font files total (regular, bold, italic, bold-italic)
Preload critical font files
Consider system font stack for body text

System font stacks:

/* Modern system fonts — zero network cost */
font-family: system-ui, -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, sans-serif;

/* Monospace */
font-family: ui-monospace, 'Cascadia Code', 'Source Code Pro', Menlo, monospace;

Phase 5: Third-Party Script Management

Impact Assessment

third_party_audit:
  - script: "Google Analytics 4"
    size_kb: 45
    blocks_render: false
    loads_more_scripts: true
    total_impact_kb: 90
    essential: true
    mitigation: "gtag async, delay until interaction"
    
  - script: "Intercom chat widget"
    size_kb: 200
    blocks_render: false
    loads_more_scripts: true
    total_impact_kb: 450
    essential: false
    mitigation: "Load on scroll/click, not page load"

Third-party loading strategies:

// Strategy 1: Load on interaction
document.addEventListener('scroll', () => {
  loadThirdParty('chat-widget.js');
}, { once: true });

// Strategy 2: Load after page is idle
requestIdleCallback(() => {
  loadThirdParty('analytics.js');
});

// Strategy 3: Facade pattern (show placeholder until needed)
chatButton.addEventListener('click', () => {
  loadThirdParty('intercom.js').then(() => Intercom('show'));
});

Rules:

Audit ALL third-party scripts quarterly
Every script needs a business justification
If a script loads >100KB, it needs a loading strategy
Self-host what you can (fonts, analytics alternatives)
Use rel="noopener" on all external links

Phase 6: Mobile Performance

Mobile-Specific Optimization

Targets (mobile on 4G):

Metric	Good	Needs Work	Poor
FCP	< 1.8s	1.8-3.0s	> 3.0s
LCP	< 2.5s	2.5-4.0s	> 4.0s
TBT	< 200ms	200-600ms	> 600ms
CLS	< 0.1	0.1-0.25	> 0.25
INP	< 200ms	200-500ms	> 500ms

Mobile-specific checklist:

Viewport meta tag present
Touch targets ≥ 48×48px
No horizontal scroll
Images responsive (srcset + sizes)
JS budget < 300KB (compressed) on mobile
Critical CSS < 14KB (fits in first TCP round trip)
Avoid complex CSS (heavy animations, large box-shadows)

Phase 7: Performance Budget

Setting Budgets

performance_budget:
  metrics:
    lcp_ms: 2500
    fcp_ms: 1800
    tbt_ms: 200
    cls: 0.1
    inp_ms: 200
    
  resources:
    total_kb: 1500
    js_kb: 350
    css_kb: 80
    images_kb: 800
    fonts_kb: 100
    
  requests:
    total: 60
    third_party: 15
    
  lighthouse:
    performance: 90
    accessibility: 90
    best_practices: 90
    seo: 90

Budget enforcement rules:

Any PR that increases JS by >10KB needs justification
LCP regression > 200ms blocks deploy
Monthly Lighthouse audit — track trend
Per-route budgets for SPAs (homepage stricter than admin)

Budget Monitoring Template

# Weekly performance check
date: "YYYY-MM-DD"
url: ""
device: "mobile"

scores:
  lighthouse: null
  lcp: null
  fcp: null
  tbt: null
  cls: null

trend: "improving | stable | degrading"
regressions: []
actions: []

Phase 8: Performance Scoring Rubric

Rate the site 0-100:

Dimension	Weight	0-2	3-4	5
Core Web Vitals	25%	All red	Mixed	All green
Page weight	15%	>5MB	2-5MB	<2MB
Caching strategy	15%	None	Partial	Full with immutable
Render path	15%	Multiple blockers	Some optimized	Clean critical path
Image optimization	10%	Unoptimized	Partially	WebP/AVIF + responsive
JavaScript health	10%	>1MB, no splitting	Some splitting	<350KB, code-split
Third-party control	5%	Unmanaged	Some deferred	All managed + budgeted
Mobile experience	5%	Desktop-only	Responsive	Mobile-first optimized

Score interpretation:

90-100: Elite. Maintain and iterate.
70-89: Good. Fix the weakest dimension.
50-69: Needs work. Follow Phase 3 playbooks.
<50: Critical. Start with server + render-blocking fixes.

Phase 9: Common Architectures — Quick Wins

Next.js / React

Use next/image (auto WebP, lazy, blur placeholder)
Enable ISR or SSG for static pages
Use dynamic() for heavy components
Check bundle with @next/bundle-analyzer
Middleware for edge caching

WordPress

Page cache plugin (WP Super Cache, W3 Total Cache)
Image optimization (ShortPixel, Imagify)
Disable unused plugins (each adds JS+CSS)
Use a CDN plugin
Consider static generation (Simply Static)

SPA (React/Vue/Svelte)

Route-based code splitting (mandatory)
SSR or SSG for SEO pages
Service worker for repeat visits
Skeleton screens (not spinners)
Virtual scrolling for long lists

Static Sites

Already fast — focus on image optimization
Deploy to CDN edge (Cloudflare Pages, Netlify, Vercel)
Inline all critical CSS
Minimal JS (< 50KB)

Phase 10: Advanced Techniques

Service Worker Caching

// Cache-first for static assets
self.addEventListener('fetch', (event) => {
  if (event.request.url.match(/\.(css|js|woff2|webp|avif)$/)) {
    event.respondWith(
      caches.match(event.request).then(cached => cached || fetch(event.request))
    );
  }
});

Resource Hints for Navigation

// Predictive prefetch on hover
document.querySelectorAll('a').forEach(link => {
  link.addEventListener('mouseenter', () => {
    const prefetch = document.createElement('link');
    prefetch.rel = 'prefetch';
    prefetch.href = link.href;
    document.head.appendChild(prefetch);
  }, { once: true });
});

Performance Monitoring in Production

// Report Core Web Vitals
new PerformanceObserver((list) => {
  for (const entry of list.getEntries()) {
    // Send to analytics
    sendToAnalytics({
      metric: entry.name,
      value: entry.value,
      rating: entry.rating, // "good" | "needs-improvement" | "poor"
    });
  }
}).observe({ type: 'largest-contentful-paint', buffered: true });

Edge Cases

Infinite scroll / pagination:

Virtual scrolling for >100 items
Intersection Observer to load batches
content-visibility: auto for off-screen items
Memory management: remove far-off-screen DOM nodes

SPAs with client-side routing:

Measure soft navigations (not just initial load)
Report per-route metrics
Prefetch likely next routes
Keep route JS < 100KB each

E-commerce product pages:

Preload first product image
Lazy load review section, related products
Defer recommendation engine JS
Cache product data with stale-while-revalidate

Media-heavy sites:

Lazy load everything below fold
Use <video> not GIF (90% smaller)
Adaptive quality based on connection (Network Information API)
Progressive JPEG for large photos

Natural Language Commands

"Audit {url}" → Run full Phase 1 audit
"Fix LCP on {url}" → Phase 3C playbook
"What's slowing down {url}?" → Phase 2 diagnosis tree
"Set performance budget for {project}" → Phase 7 template
"Score {url}" → Phase 8 rubric
"Optimize images on {url}" → Phase 3C image checklist
"Reduce JavaScript on {url}" → Phase 3D JS optimization
"Fix layout shifts on {url}" → Phase 3E CLS playbook
"Mobile performance audit for {url}" → Phase 6
"Third-party script audit for {url}" → Phase 5
"Weekly performance check for {url}" → Phase 7 monitoring template
"Compare {url1} vs {url2}" → Side-by-side audit