Case Study: How I Reduced React Bundle Size by 68%

Source: Dev.to

Introduction

I worked on a B2B application where I faced a common misconception: businesses are more tolerant of poor Google Core Web Vitals and bloated bundle sizes because “users access from office PCs with fast internet.” In reality, primary users accessed the application from mobile phones with mobile internet connections. When I discovered this, optimization became critical.

Starting Point: The production bundle size was 1,542 KB (gzipped).
Target: 500 KB (following industry best practices)

Step 1: Compression – Already Optimized

The obvious first step was enabling Brotli compression, which typically saves 15–25 % compared to gzip. However, the infrastructure already had Brotli enabled, so no gains were possible here.

Step 2: Route‑Based Code Splitting

React Router supports lazy loading out of the box. I implemented dynamic imports for all routes:

// Before
import Dashboard from './Dashboard';

// After
const Dashboard = React.lazy(() => import('./Dashboard'));

Result: Bundle reduced by 37 % to 971 KB.

Step 3: Dependency Analysis & Vendor Splitting

Using Webpack Bundle Analyzer, I discovered heavy third‑party libraries dominating the bundle:

• Date picker/calendar libraries
• Feature‑flag management
• Analytics SDKs
• PDF editor
• WYSIWYG editor
• File upload widget

These were imported directly across multiple modules. I created wrapper modules with dynamic imports:

// Before – Direct imports everywhere
import { DatePicker } from 'heavy-date-library';

// After – Wrapper with lazy loading
export const loadDatePicker = () =>
  import('heavy-date-library').then(module => module.DatePicker);

Result: Bundle reduced to 57 % of the original size (879 KB).

Step 4: Module Decoupling & Dependency Chains

The bundle analyzer revealed tightly coupled dependency chains. For example:

// ❌ Tightly coupled
// Module A imported Module B, which imported Module C,
// which imported a heavy utility library
// All loaded together even if only one piece was needed

// ✅ Solution: Extract shared code to independent modules
// Created small, focused modules with only necessary shared code
// Broke circular dependencies

I identified and separated these chains, creating smaller, focused modules with only the necessary shared code.

Result: Bundle at 63 % reduction (571 KB).

Step 5: Localization Optimization

The application supported several languages, but all translations loaded synchronously regardless of user preference.

Before: All language files bundled together (≈ 120 KB of JSON).
After: Only the user’s selected language loads initially:

// Dynamic locale loading
const loadLocale = (locale) =>
  import(`./locales/${locale}.json`);

Final Result: 493 KB – a 68 % reduction that beat our 500 KB target!

Key Takeaways

• Assumptions are dangerous: Never assume user context without data.
• Analyze before optimizing: Webpack Bundle Analyzer was invaluable.
• Third‑party libraries are expensive: Be strategic about heavy dependencies.
• Dependency chains matter: Tight coupling creates bundle bloat.
• Localization can be heavy: Load language files dynamically.