React vs. Svelte: A Detailed Performance Comparison - VoidBin Blog

Introduction

The JavaScript ecosystem is constantly evolving, with new frameworks and libraries emerging to address different developer needs. Among these, React and Svelte have gained significant traction—React for its flexibility and large ecosystem, and Svelte for its compile-time optimizations and simplicity. But how do they compare in terms of performance?

This post dives deep into the performance characteristics of React and Svelte, analyzing key metrics such as bundle size, rendering efficiency, and runtime overhead. We'll also provide practical examples to illustrate these differences and help you decide which framework might be best for your project.

Bundle Size and Initial Load Performance

One of the most noticeable differences between React and Svelte is their bundle size. React is a runtime-based library, meaning it includes a virtual DOM and reconciliation logic that runs in the browser. Svelte, on the other hand, compiles components into highly optimized vanilla JavaScript at build time, eliminating the need for a large runtime.

React Bundle Size

A basic React app with create-react-app typically starts at around 140KB (minified + gzipped). This includes React, ReactDOM, and the required runtime. Additional optimizations (like code splitting) can help, but the baseline remains higher than Svelte.

Svelte Bundle Size

A comparable Svelte app often weighs under 10KB (minified + gzipped). Since Svelte compiles away the framework code, only the necessary JavaScript is included in the final bundle.

Example: A Simple Counter Component

React:

import React, { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <div>
      <button onClick={() => setCount(count + 1)}>Increment</button>
      <p>Count: {count}</p>
    </div>
  );
}

Svelte:

<script>
  let count = 0;
  function increment() {
    count += 1;
  }
</script>

<button on:click={increment}>Increment</button>
<p>Count: {count}</p>

Takeaway: Svelte's compile-time approach results in smaller bundles, which can lead to faster initial load times—especially critical for mobile users or slower networks.

Rendering Performance and Updates

Another critical performance factor is how efficiently each framework handles DOM updates.

React's Virtual DOM

React uses a virtual DOM to minimize direct DOM manipulations. While effective, this introduces overhead since React must:

Create a virtual representation of the UI.
Diff the new and old virtual DOMs.
Apply the minimal set of changes to the real DOM.

This reconciliation process can become a bottleneck in complex applications with frequent updates.

Svelte's Compile-Time Optimizations

Svelte skips the virtual DOM entirely. Instead, it compiles components into precise DOM update instructions. This means:

No diffing overhead.
Direct, surgical updates to the DOM.
Less runtime work.

Benchmark Example: Updating a Large List
In stress tests where a large list (e.g., 10,000 items) is updated, Svelte often outperforms React due to its lack of virtual DOM reconciliation.

React List Update:

import React, { useState } from 'react';

function LargeList() {
  const [items, setItems] = useState(Array(10000).fill(0));

  function updateItems() {
    setItems(items.map((item, i) => item + 1));
  }

  return (
    <div>
      <button onClick={updateItems}>Update All</button>
      <ul>
        {items.map((item, i) => (
          <li key={i}>{item}</li>
        ))}
      </ul>
    </div>
  );
}

Svelte List Update:

<script>
  let items = Array(10000).fill(0);
  function updateItems() {
    items = items.map((item, i) => item + 1);
  }
</script>

<button on:click={updateItems}>Update All</button>
<ul>
  {#each items as item, i}
    <li>{item}</li>
  {/each}
</ul>

Takeaway: Svelte's direct DOM updates can lead to better performance in scenarios with frequent or large-scale UI changes.

Developer Experience and Ecosystem

While performance is crucial, developer experience and ecosystem support also play a role in choosing a framework.

React's Strengths

Mature ecosystem: Rich library support (e.g., Redux, React Router).
Flexibility: Works with various state management solutions.
Community: Extensive resources, tutorials, and third-party tools.

Svelte's Strengths

Simpler syntax: Less boilerplate, more intuitive reactivity.
Built-in features: Stores, animations, and transitions are included.
Easier learning curve: Less abstraction than React's hooks and virtual DOM.

Example: State Management

React (with Context API):

import React, { createContext, useContext, useState } from 'react';

const CountContext = createContext();

function App() {
  const [count, setCount] = useState(0);
  return (
    <CountContext.Provider value={{ count, setCount }}>
      <ChildComponent />
    </CountContext.Provider>
  );
}

function ChildComponent() {
  const { count, setCount } = useContext(CountContext);
  return <button onClick={() => setCount(count + 1)}>Count: {count}</button>;
}

Svelte (with Stores):

<script>
  import { writable } from 'svelte/store';
  export const count = writable(0);
</script>

<button on:click={() => $count += 1}>Count: {$count}</button>

Takeaway: React offers more flexibility for large-scale apps, while Svelte reduces boilerplate and complexity for smaller to medium projects.

Conclusion

React and Svelte take fundamentally different approaches to building web applications, each with its own performance trade-offs:

React excels in large-scale applications with its mature ecosystem and flexibility but carries a runtime overhead.
Svelte offers superior performance in many cases due to its compile-time optimizations and smaller bundle size, making it ideal for performance-critical applications.

Final Recommendation:

Choose React if you need a battle-tested framework with extensive tooling and scalability.
Choose Svelte if performance, simplicity, and smaller bundles are top priorities.

Both frameworks have their place in modern web development, and the best choice depends on your project's specific requirements.