SvelteKit Performance Optimization: AI-Optimized Technical Reference

Critical Performance Thresholds

Bundle Size Breaking Points

• moment.js: 67KB - replace with date-fns immediately
• lodash: 70KB+ - write custom utility functions
• crypto libraries: 180KB+ - move to server-side processing
• Total bundle >2MB: causes user abandonment on 3G connections
• Mobile 3G load time >3s: 32% bounce rate increase
• Mobile 3G load time >5s: 90% bounce rate increase

Core Web Vitals Failure Thresholds

• LCP >2.5s: Google search ranking penalty
• FID >100ms: Users perceive unresponsiveness
• CLS >0.1: Layout jumps cause user frustration
• FCP >1.8s: Users notice significant slowness
• TBT >300ms: Interface feels laggy

Configuration That Works in Production

Server Load Functions Setup

// CRITICAL: Use +page.server.ts (not +page.ts) for heavy dependencies
// This keeps libraries on server, only sends results to client

// src/routes/orders/[id]/+page.server.ts
import moment from 'moment'; // 67KB stays on server

export async function load({ params }) {
  const order = await getOrder(params.id);
  return {
    formattedDate: moment(order.date).format('MMMM Do, YYYY')
  };
}

Bundle Analysis Configuration

// vite.config.js - Essential for identifying bloat
import { visualizer } from 'rollup-plugin-visualizer';

export default {
  plugins: [
    sveltekit(),
    visualizer({
      filename: 'bundle-analysis.html',
      gzipSize: true,
      template: 'treemap' // Visual identification of largest chunks
    })
  ]
}

Production Caching Headers

// src/hooks.server.js - Aggressive caching for repeat visitors
export async function handle({ event, resolve }) {
  const response = await resolve(event);
  
  // Static assets: cache forever
  if (event.url.pathname.startsWith('/_app/')) {
    response.headers.set(
      'Cache-Control', 
      'public, max-age=31536000, immutable'
    );
  }
  
  // API calls: cache briefly, revalidate in background
  if (event.url.pathname.startsWith('/api/')) {
    response.headers.set(
      'Cache-Control', 
      'public, max-age=60, s-maxage=300, stale-while-revalidate=86400'
    );
  }
  
  return response;
}

Resource Requirements and Time Investment

Technique	Bundle Size Reduction	Implementation Time	Skill Level	Breaking Risk	Real Impact
Server Load Functions	30-50%	15-30 min per route	Beginner	Very Low	High - Immediate mobile improvement
Bundle Analysis	Identifies problems	5 minutes setup	Beginner	None	High - Shows exact bloat sources
Remove moment.js	-67KB instantly	30 minutes	Beginner	None	High - No-brainer replacement
Dynamic Imports	Variable	1-2 hours per component	Intermediate	High - Loading state complexity	Medium - UX trade-offs
Image Optimization	60-80% page weight	2-4 hours total	Beginner	Low	Very High - Massive LCP improvement
Edge Deployment	Latency reduction	1-2 hours setup	Intermediate	Medium - Config issues	High for global users

Critical Warnings and Failure Modes

Memory Leak Prevention

// WRONG: Memory leak waiting to happen
let interval;
onMount(() => {
  interval = setInterval(() => {
    // Memory leak - interval never cleared
  }, 1000);
});

// RIGHT: Always clean up subscriptions
let interval;
onMount(() => {
  interval = setInterval(() => {
    // Safe operation
  }, 1000);
  
  return () => {
    clearInterval(interval); // CRITICAL: Prevents server crashes
  };
});

Windows Development Issues

• WSL2 filesystem crossing: Put projects in /home/user/ not /mnt/c/ - crossing boundaries kills performance
• Node.js v18.12.0: Memory leak in WSL2 crashes dev server every 2 hours
• Node.js v18.15.0: Breaks ES modules
• Node.js v18.14.2: Works but crashes VS Code daily
• Windows Defender: Scans every file change, add project to exclusion list

Scaling Limitations

• 500+ components: VS Code language server consumes 8GB RAM, crashes every 20 minutes
• 800+ components: Daily VS Code crashes, TypeScript checking becomes unusable
• Build time degradation: Exponential increase with project size
• Memory requirements: --max-old-space-size=8192 becomes mandatory

Production Deployment Configurations

Vercel (Easiest Setup)

• Pros: Zero-config deployment, built-in image optimization, executive-friendly analytics
• Cons: Expensive at scale
• Best for: Teams prioritizing speed over cost

Cloudflare Pages (Most Cost-Effective)

• Pros: Cheapest edge deployment, excellent performance
• Cons: Complex setup, assumes Cloudflare knowledge
• Best for: Cost-conscious teams with DevOps expertise

Netlify (Balanced Option)

• Pros: Easier than Cloudflare, cheaper than Vercel, responsive support
• Cons: Occasional build system quirks
• Best for: Mid-size teams needing reliability

Monitoring and Alerting Setup

Lighthouse CI Integration

# .github/workflows/lighthouse.yml - Prevents performance regressions
- name: Lighthouse CI
  run: |
    npm run build
    npx @lhci/cli@0.12.x autorun
  env:
    LHCI_GITHUB_APP_TOKEN: ${{ secrets.LHCI_GITHUB_APP_TOKEN }}

Real User Monitoring

// Track actual user performance (not synthetic tests)
import { getCLS, getFID, getFCP, getLCP, getTTFB } from 'web-vitals';

getCLS(console.log);
getFID(console.log);
getFCP(console.log);
getLCP(console.log);
getTTFB(console.log);

Bundle Size Emergency Fixes

Immediate Wins (< 1 hour)

1. Remove moment.js → Switch to date-fns (-67KB)
2. Remove full lodash → Use individual functions (-50KB+)
3. Audit crypto libraries → Move to server functions (-180KB+)
4. Remove unused icon imports → Import specific icons only

Dynamic Import Guidelines

• Only use for: Below-the-fold components, admin panels, rarely-used features
• Avoid for: Above-the-fold content, frequently-accessed components
• User experience cost: Loading spinners create perceived slowness

Image Optimization Implementation

Enhanced Image Component

// Automatic WebP/AVIF conversion, lazy loading, responsive sizing
<enhanced:img src="hero.jpg" alt="Hero" />

Third-Party Solutions

• Cloudinary: Complex transformations, real-time resizing
• ImageKit: Cost-effective alternative
• Vercel Image Optimization: Built-in if using Vercel

Business Impact Calculations

Revenue Impact Formula

• Bounce rate increase: 1s to 3s load time = 32% increase
• Abandonment cost: Users × Abandonment Rate × Average Order Value
• Example: 1000 monthly users × 32% increase × $50 AOV = $16,000 monthly loss
• SEO penalty: LCP >2.5s reduces organic search rankings

Performance Budget Thresholds

• JavaScript bundle: <500KB gzipped
• Images: <2MB total page weight
• LCP target: <2.5s on 3G
• Memory usage: <100MB peak (mobile compatibility)

Troubleshooting Common Issues

"Moved to .server.ts but still slow"

• Check for dynamic imports in server functions (ships to client)
• Verify imports are at top level, not inside functions
• Ensure heavy libraries aren't imported in components

"Bundle analysis shows no obvious bloat"

• Check for multiple versions of same library
• Audit polyfills and compatibility libraries
• Review tree-shaking effectiveness

"Performance good locally, terrible in production"

• Test on real mobile devices with throttled connections
• Check CDN configuration and geographic distribution
• Verify caching headers are set correctly
• Monitor memory leaks with long-running processes

Decision Matrix for Optimization Priorities

High Impact, Low Effort

1. Server load functions for heavy libraries
2. Remove moment.js/lodash
3. Enhanced image component
4. Basic caching headers

Medium Impact, Medium Effort

1. Edge deployment setup
2. Dynamic imports for large components
3. Bundle analysis and cleanup
4. Performance monitoring integration

High Effort, Variable Impact

1. Complete architecture refactoring
2. Custom lazy loading implementation
3. Advanced caching strategies
4. Multi-region deployment

Reality Check: What Won't Save You

Common Misconceptions

• Lighthouse 100 scores: Don't correlate with real user experience
• Preloading everything: Makes initial load slower, not faster
• Micro-optimizations: Moving from 98KB to 95KB won't be noticeable
• Dynamic imports everywhere: Loading states harm UX more than bundle size

When to Consider Architecture Changes

• >500 components: SvelteKit scaling issues become apparent
• Complex data flows: Consider state management refactoring
• Multiple teams: Monorepo might require splitting
• Enterprise requirements: May need framework migration