Fix Your Slow-Ass SvelteKit App Performance

Your SvelteKit app loads like molasses and you know it. Users bail on checkout because it takes forever. Management keeps asking why the "modern" framework is slower than the old PHP site they're paying you to replace.

I spent 6 months debugging performance disasters in production SvelteKit apps. Here's the real shit that breaks performance and the fixes that actually work.

Bundle Bloat Will Murder Your App

The biggest issue? Every dependency ships to the browser. moment.js alone is 67KB. Add lodash (70KB), some crypto library (180KB+), and suddenly you're shipping 2MB+ to mobile users on 3G connections.

Server Load Functions Actually Work

Use server `load` functions instead of doing heavy lifting client-side:

// src/routes/orders/[id]/+page.server.ts
// This runs on server, not client
import moment from 'moment'; 

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

Move date formatting, crypto operations, and data transformation to the server. Ship the result, not the library. This reduces your bundle size and improves Time to Interactive for users.

Bundle Analysis Will Show You The Damage

Install rollup-plugin-visualizer to see what's killing your bundle size:

npm install --save-dev rollup-plugin-visualizer

// vite.config.js
import { visualizer } from 'rollup-plugin-visualizer';

export default {
  plugins: [
    sveltekit(),
    visualizer({
      filename: 'bundle-analysis.html',
      gzipSize: true
    })
  ]
}

Run npm run build and open bundle-analysis.html. The biggest blocks are your biggest problems. Usually it's date libraries, UI frameworks you're barely using, or some massive polyfill.

Bundle visualization shows exactly what's eating your bandwidth - no guesswork needed. Check Bundle Phobia before adding any new dependency to understand its impact on performance.

Windows Development is Pain

Hot reload on Windows is slower than continental drift. WSL2 helps but brings its own special hell.

Node.js v18.12.0 has a memory leak in WSL2 that kills your dev server every 2 hours. Node v18.15.0 fixed it but broke ES modules for some fucking reason. I'm running v18.14.2 which works but crashes VS Code every morning. Pick your poison.

If you're stuck on Windows:

• Use WSL2, not native Windows development
• Put your project in Linux filesystem (/home/user/ not /mnt/c/) - seriously, crossing filesystem boundaries kills performance
• Accept that file watching will be slow and sometimes just stops working
• Install dependencies inside WSL2, not Windows - mixing package managers breaks everything
• Consider switching to Mac for development if debugging Windows file system issues costs more than the hardware

Also, Windows Defender scans every file change which makes Vite's hot reload even slower. Adding your project folder to the exclusion list helps but your security team will hate you.

Performance Monitoring That Actually Matters

Stop obsessing over Lighthouse scores and monitor real users. Core Web Vitals matter for SEO and user experience:

• LCP > 2.5s: Google ranking penalty
• FID > 100ms: Feels unresponsive
• CLS > 0.1: Layout jumps annoy users

Use web-vitals to track real performance from actual users:

npm install web-vitals

// src/app.html - track real user metrics
import { getCLS, getFID, getFCP, getLCP, getTTFB } from 'web-vitals';

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

Lighthouse CI catches performance regressions before production. Add it to your CI pipeline and it'll fail builds when performance tanks. This prevents shipping slow code to users.

That covers the theory behind performance optimization. Now let's talk about what actually works in practice. I've tested dozens of performance techniques over the years - some are game-changers, others are complete wastes of time. Here's what really moves the needle...

Production Performance Disasters and How to Fix Them

So you followed the basic advice - you know bundle bloat kills performance, you've heard about server load functions. But your SvelteKit app still works fine locally and dies in production. Memory leaks crash the server after 3 days uptime. Bundle sizes murder mobile users despite your best efforts.

Here's the deeper production disasters that break real apps and how I actually fixed them.

The Bundle Analysis That Saved My Ass

rollup-plugin-visualizer showed me the ugly truth about my bundle. 67KB of moment.js. 120KB of lodash. 200KB of some encryption library I forgot I installed. Each page load was downloading half a megabyte of JavaScript that could run on the server.

// vite.config.js - This generates the treemap visualization
import { visualizer } from 'rollup-plugin-visualizer';

export default defineConfig({
  plugins: [
    sveltekit(),
    visualizer({
      filename: 'bundle-size.html',
      open: true,
      gzipSize: true,
      template: 'treemap' // Shows biggest chunks visually
    })
  ]
});

Run npm run build and it opens an interactive treemap. The biggest colored blocks are your biggest problems. Mine looked like a fucking rainbow because every page was importing massive libraries.

Moved moment.js, lodash, and the crypto stuff to server load functions. Bundle size dropped significantly. Mobile users noticed the difference immediately.

Server Load Functions: Move Heavy Shit to the Server

The difference between `+page.ts` and `+page.server.ts` matters more than I realized. Universal load functions ship everything to the client. Server load functions keep dependencies on the server and only send results.

// WRONG: Universal load (+page.ts)
// This ships 67KB of moment.js to every user
import moment from 'moment'; 
export const load = () => ({
  todayFormatted: moment().format('MMMM Do, YYYY')
});

// RIGHT: Server load (+page.server.ts)  
// moment.js stays on server, only formatted string goes to client
import moment from 'moment';
export const load = () => ({
  todayFormatted: moment().format('MMMM Do, YYYY')
});

Moving from universal to server load functions takes some work but reduces bundle size significantly. Every mobile user on 3G will notice faster loading.

Dynamic Imports: The Double-Edged Sword

Lazy loading heavy components sounds great until you deal with loading states and error handling everywhere. Users see spinners constantly if you're not careful.

// Don't do this everywhere - users hate loading spinners
{#await import('$lib/HeavyChart.svelte')}
  <div class=\"skeleton\">Loading chart...</div>
{:then module}
  <svelte:component this={module.default} {data} />
{:catch error}
  <div class=\"error\">Chart failed to load</div>
{/await}

I use dynamic imports sparingly - only for components that:

1. Are below the fold
2. Users rarely interact with (admin panels, settings pages)
3. Are genuinely large (>50KB with dependencies)

For everything else, the loading state UX is worse than the bundle size cost.

Caching: The Fix That Keeps on Fixing

Production performance isn't just about bundle size. Repeat visitors need aggressive caching or they'll still get shitty performance.

// src/hooks.server.js - Cache everything intelligently
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;
}

This dramatically improves repeat visit performance. Pages load instantly for returning users with proper caching headers.

Memory Leaks: The Silent App Killer

Memory Leak Detection and Analysis

The heap memory errors turned out to be memory leaks from subscriptions I never cleaned up. Svelte DevTools showed components staying in memory long after unmounting.

// WRONG: Memory leak waiting to happen
import { browser } from '$app/environment';
let interval;

onMount(() => {
  if (browser) {
    interval = setInterval(() => {
      // Do something every second
    }, 1000);
  }
});

// RIGHT: Clean up your shit
import { browser } from '$app/environment';
let interval;

onMount(() => {
  if (browser) {
    interval = setInterval(() => {
      // Do something every second
    }, 1000);
  }
  
  return () => {
    clearInterval(interval);
  };
});

Clean up subscriptions and intervals to prevent memory leaks. This keeps your app stable and prevents crashes in production.

The Performance Monitoring That Actually Helps

Chrome DevTools is great for development but useless for understanding real user performance. Lighthouse CI in your CI/CD pipeline catches performance regressions before they hit production.

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

This fails the build if performance drops below thresholds. Saved my ass twice when someone added libraries without thinking about mobile performance.

For production monitoring, I use New Relic Browser. Sets up alerts when Core Web Vitals go to shit so I know about problems before customers start complaining.