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Svelte Performance Optimization

Master Svelte's compile-time advantages, reactive statements, SvelteKit features, and optimization strategies for blazing-fast apps.

Why Svelte is Already Fast

Svelte is a compiler that converts your components into highly efficient JavaScript at build time, eliminating the virtual DOM overhead. This gives Svelte a performance advantage out of the box, but understanding its features can unlock even better performance and smaller bundle sizes.

Compile-Time Magic

No runtime overhead, compiled to vanilla JavaScript.

Reactive Statements

Automatic dependency tracking with reactive declarations.

Small Bundle Size

No framework runtime, only your code ships to browsers.

SvelteKit SSR

Server-side rendering for better initial load performance.

1. Leverage Reactive Statements

Svelte's reactivity is compile-time optimized, but proper usage maximizes performance:

Reactive Declarations

html
<script>
  let count = 0;


  // Reactive statement - only recomputes when count changes
  $: doubled = count * 2;


  // Reactive block
  $: {
    console.log(`Count is ${count}`);
    console.log(`Doubled is ${doubled}`);
  }


  // Conditional reactive statements
  $: if (count > 10) {
    console.log('Count exceeded 10');
  }
</script>


<button on:click={() => count++}>
  Count: {count}
</button>
<p>Doubled: {doubled}</p>

Avoid Expensive Computations

javascript
<script>
  let items = [];


  // Bad - recomputes on every update
  $: total = items.reduce((sum, item) => sum + expensiveCalc(item), 0);


  // Good - memoize expensive calculations
  import { derived } from 'svelte/store';
  const itemsStore = writable(items);
  const total = derived(itemsStore, $items =>
    $items.reduce((sum, item) => sum + item.price, 0)
  );
</script>

2. Optimize Stores and State Management

Svelte stores are lightweight and efficient when used correctly:

Writable and Readable Stores

javascript
// stores.js
import { writable, derived, readable } from 'svelte/store';


export const count = writable(0);


// Derived store - only updates when dependencies change
export const doubled = derived(count, $count => $count * 2);


// Readable store for external data
export const time = readable(new Date(), set => {
  const interval = setInterval(() => {
    set(new Date());
  }, 1000);


  return () => clearInterval(interval);
});

Custom Stores

javascript
// Custom store with methods
function createCounter() {
  const { subscribe, set, update } = writable(0);


  return {
    subscribe,
    increment: () => update(n => n + 1),
    decrement: () => update(n => n - 1),
    reset: () => set(0)
  };
}


export const counter = createCounter();

Unsubscribe Automatically

javascript
<script>
  import { count } from './stores';


  // $ prefix auto-subscribes and auto-unsubscribes
  // No manual cleanup needed!
</script>


<p>Count: {$count}</p>

3. Component Optimization

Write efficient Svelte components:

Use Key Blocks for Lists

bash
<script>
  let items = [/* ... */];
</script>


<!-- Always use key for dynamic lists -->
{#each items as item (item.id)}
  <div>{item.name}</div>
{/each}

Lazy Load Components

javascript
<script>
  let HeavyComponent;


  async function loadComponent() {
    const module = await import('./HeavyComponent.svelte');
    HeavyComponent = module.default;
  }
</script>


<button on:click={loadComponent}>Load Component</button>


{#if HeavyComponent}
  <svelte:component this={HeavyComponent} />
{/if}

Immutable Data

javascript
<svelte:options immutable={true} />


<script>
  // Component only re-renders if props change (===)
  export let data;
</script>

4. SvelteKit Performance Features

Leverage SvelteKit's built-in optimizations:

Page Load Strategies

javascript
// +page.js
export const load = async ({ fetch }) => {
  const res = await fetch('/api/data');
  const data = await res.json();
  return { data };
};


// Prerender at build time
export const prerender = true;


// Or use server-side rendering
export const ssr = true;


// Client-side only rendering
export const csr = true;

Preloading Data

html
<!-- Preload on hover -->
<a href="/about" data-sveltekit-preload-data="hover">
  About
</a>


<!-- Preload on tap (mobile) -->
<a href="/contact" data-sveltekit-preload-data="tap">
  Contact
</a>

Route Configuration

javascript
// svelte.config.js
export default {
  kit: {
    prerender: {
      entries: ['*'],
      crawl: true
    },
    csp: {
      mode: 'auto'
    }
  }
};

5. Bundle Optimization

Minimize bundle size with Vite configuration:

Vite Configuration

javascript
// vite.config.js
import { sveltekit } from '@sveltejs/kit/vite';


export default {
  plugins: [sveltekit()],
  build: {
    rollupOptions: {
      output: {
        manualChunks: {
          vendor: ['svelte']
        }
      }
    },
    minify: 'terser',
    terserOptions: {
      compress: {
        drop_console: true
      }
    }
  }
};

Analyze Bundle

javascript
# Install analyzer
npm install -D rollup-plugin-visualizer


# Add to vite.config.js
import { visualizer } from 'rollup-plugin-visualizer';


export default {
  plugins: [
    sveltekit(),
    visualizer({ open: true })
  ]
};

6. Svelte-Specific Best Practices

Additional optimization tips:

  • •Use bind: sparingly - it adds overhead
  • •Avoid inline event handlers in loops
  • •Use svelte:window for global event listeners
  • •Prefer CSS for animations over JavaScript when possible
  • •Use tick() for DOM updates timing control
  • •Enable hydration for SSR with hydratable: true

Analyze Your Svelte App

Get Svelte-specific optimization recommendations powered by AI.

Test Your Svelte Site