Performance Optimization

Techniques and best practices for optimizing Svelte application performance including reactivity, rendering, and bundle size

Svelte is fast by default, but understanding performance optimization techniques helps you build even faster applications.

Reactivity Optimization

Use $state Sparingly

Only make variables reactive when necessary:

<script>
  // ❌ Unnecessary reactivity
  let apiUrl = $state('https://api.example.com');
  let timeout = $state(5000);
  
  // ✅ Good: These don't need to be reactive
  const apiUrl = 'https://api.example.com';
  const timeout = 5000;
  
  // ✅ Only state that actually changes needs $state
  let data = $state(null);
</script>

Reactive state has overhead. Use plain variables for constants and values that never change.

Choose Between $state and $state.raw

For large objects that are only reassigned (not mutated), use $state.raw:

<script>
  // ❌ Expensive: Deep reactivity proxy for large object
  let apiResponse = $state({
    users: [...1000 users],
    metadata: {...}
  });
  
  // ✅ Better: No deep proxying, just reassignment tracking
  let apiResponse = $state.raw({
    users: [...1000 users],
    metadata: {...}
  });
  
  async function fetchData() {
    // Reassigning works fine
    apiResponse = await fetch('/api/data').then(r => r.json());
  }
</script>

Use $state.raw when:

• Working with large API responses
• Data is replaced wholesale, not mutated
• Objects are frequently reassigned

Use $state when:

• Mutating nested properties (user.name = 'Alice')
• Need fine-grained reactivity
• Working with small objects

Prefer $derived over $effect

Compute values with $derived, not $effect:

<script>
  let count = $state(0);
  
  // ❌ Bad: Using effect to compute derived value
  let doubled;
  $effect(() => {
    doubled = count * 2;
  });
  
  // ✅ Good: Use $derived for computed values
  let doubled = $derived(count * 2);
</script>

$derived is more efficient and clearer than $effect for computing values from state.

Avoid Reactive Statement Overhead

In legacy mode, reactive statements ($:) run more often than necessary:

<script>
  let items = $state([...]);
  
  // Svelte 4 style - runs on any dependency change
  $: filteredItems = items.filter(item => item.active);
  
  // ✅ Svelte 5 - more efficient
  let filteredItems = $derived(items.filter(item => item.active));
</script>

Always use runes mode ($state, $derived, $effect) for better performance. Avoid legacy reactive statements.

Rendering Optimization

Use Keyed Each Blocks

Always provide keys in {#each} blocks for efficient updates:

<!-- ❌ Bad: Svelte re-renders all items -->
{#each items as item}
  <div>{item.name}</div>
{/each}

<!-- ✅ Good: Svelte surgically updates only changed items -->
{#each items as item (item.id)}
  <div>{item.name}</div>
{/each}

Performance impact:

• Without key: O(n) - Updates all existing DOM nodes
• With key: O(log n) - Moves/inserts/removes specific nodes

Avoid Index as Key

<script>
  let items = $state(['A', 'B', 'C']);
  
  function removeFirst() {
    items = items.slice(1); // Now ['B', 'C']
  }
</script>

<!-- ❌ Bad: Index changes when items are removed -->
{#each items as item, i (i)}
  <div>{item}</div>
{/each}
<!-- After removal: 'B' gets index 0, 'C' gets index 1 -->
<!-- Svelte thinks these are NEW items -->

<!-- ✅ Good: Use stable identifier -->
{#each items as item (item)}
  <div>{item}</div>
{/each}

Minimize Component Re-renders

Prevent unnecessary work by isolating reactive dependencies:

<!-- ❌ Bad: Entire component re-renders when count changes -->
<script>
  let count = $state(0);
  let expensiveData = $state([...large dataset]);
</script>

<button onclick={() => count++}>{count}</button>
<ExpensiveList items={expensiveData} />

<!-- ✅ Better: Isolate reactive scope -->
<script>
  let count = $state(0);
  let expensiveData = $state([...large dataset]);
</script>

<div>
  <button onclick={() => count++}>{count}</button>
</div>
<ExpensiveList items={expensiveData} />

Component Design Patterns

Extract Expensive Logic

Move expensive computations to dedicated components:

<!-- ❌ Bad: All logic in one component -->
<script>
  let items = $state([...]);
  let filter = $state('');
  
  let processed = $derived(
    items
      .filter(item => item.name.includes(filter))
      .map(item => expensiveTransform(item))
      .sort((a, b) => a.score - b.score)
  );
</script>

{#each processed as item (item.id)}
  <div>{item.name}</div>
{/each}

<!-- ✅ Better: Separate concerns -->
<script>
  import ItemList from './ItemList.svelte';
  
  let items = $state([...]);
  let filter = $state('');
</script>

<ItemList {items} {filter} />

Use Event Delegation

For many similar elements, use event delegation:

<!-- ❌ Bad: N event listeners -->
{#each items as item (item.id)}
  <button onclick={() => handleClick(item.id)}>
    {item.name}
  </button>
{/each}

<!-- ✅ Better: One event listener -->
<div onclick={(e) => {
  const id = e.target.dataset.id;
  if (id) handleClick(id);
}}>
  {#each items as item (item.id)}
    <button data-id={item.id}>{item.name}</button>
  {/each}
</div>

Avoid Inline Object/Array Creation

<!-- ❌ Bad: Creates new object every render -->
<Component style={{ color: 'red', fontSize: '16px' }} />

<!-- ✅ Good: Reuses same object -->
<script>
  const buttonStyle = { color: 'red', fontSize: '16px' };
</script>
<Component style={buttonStyle} />

Bundle Size Optimization

Code Splitting with Dynamic Imports

Lazy load components that aren’t immediately needed:

<script>
  let showModal = $state(false);
  let Modal;
  
  async function openModal() {
    if (!Modal) {
      const module = await import('./Modal.svelte');
      Modal = module.default;
    }
    showModal = true;
  }
</script>

<button onclick={openModal}>Open Modal</button>

{#if showModal && Modal}
  <Modal />
{/if}

Tree Shaking

Import only what you need:

// ❌ Bad: Imports entire library
import _ from 'lodash';
const result = _.debounce(fn, 300);

// ✅ Good: Imports only debounce
import { debounce } from 'lodash-es';
const result = debounce(fn, 300);

Analyze Bundle Size

Use tools to find optimization opportunities:

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

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

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

Advanced Optimizations

Virtual Lists for Large Datasets

For thousands of items, render only visible rows:

<script>
  import { VirtualList } from 'svelte-virtual-list';
  
  let items = $state([...10000 items]);
</script>

<VirtualList items={items} let:item>
  <div>{item.name}</div>
</VirtualList>

Debounce Expensive Operations

<script>
  import { debounce } from './utils';
  
  let searchTerm = $state('');
  let results = $state([]);
  
  const search = debounce(async (term) => {
    results = await fetch(`/api/search?q=${term}`).then(r => r.json());
  }, 300);
  
  $effect(() => {
    if (searchTerm) search(searchTerm);
  });
</script>

<input bind:value={searchTerm} placeholder="Search..." />

Memoize Complex Computations

<script>
  let data = $state([...]);
  
  // Cache expensive computation
  let cache = new Map();
  
  let processed = $derived.by(() => {
    const key = JSON.stringify(data);
    if (cache.has(key)) return cache.get(key);
    
    const result = expensiveComputation(data);
    cache.set(key, result);
    return result;
  });
</script>

Image and Asset Optimization

Lazy Load Images

<img 
  src="placeholder.jpg" 
  data-src="high-res.jpg" 
  loading="lazy"
  alt="Description"
/>

Use Modern Formats

<picture>
  <source srcset="image.avif" type="image/avif" />
  <source srcset="image.webp" type="image/webp" />
  <img src="image.jpg" alt="Fallback" />
</picture>

Profiling and Measurement

Use Browser DevTools

Use $inspect.trace

Debug reactive dependencies:

<script>
  let items = $state([...]);
  
  $effect(() => {
    $inspect.trace('items effect'); // First line of effect
    console.log('Items changed:', items.length);
  });
  // Logs what triggered this effect to re-run
</script>

Best Practices Checklist

Common Performance Pitfalls

Avoid these patterns:

• Using array index as {#each} key
• Creating objects/arrays in templates: style={{ color }}
• Making everything $state “just in case”
• Using $effect for derived values
• Not profiling before optimizing
• Premature optimization without measurements

Svelte is already very fast. Focus on correctness first, then optimize bottlenecks identified through profiling.