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Gouranga Das Samrat
Gouranga Das Samrat

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How We Cut Our React Bundle Size by 40% with Smart Code-Splitting

#javascript #performance #react

The Problem: A Bloated Bundle Killing Performance

Our React app had grown to a 1.8MB initial bundle — causing:

  • 🐢 3.5s+ Time-to-Interactive (TTI) on mobile
  • 📉 30% bounce rate from slow-loading pages
  • 🔍 Google Lighthouse score: 52/100 # Before:
  • main.chunk.js 1.4MB
  • vendor.chunk.js 400KB 

The Solution: Strategic Code-Splitting
--------------------------------------

1. Route-Based Splitting (Lazy Loading)
----------------------------------------

Changed this:
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import Home from './Home';
import Dashboard from './Dashboard';


To this:
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const Home = React.lazy(() => import('./Home'));
const Dashboard = React.lazy(() => import('./Dashboard'));


Impact:

*   Initial load ↓ 28% (removed 500KB unused routes)

2. Component-Level Splitting
-----------------------------

For heavy components (e.g., charts, PDF viewers):
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const Chart = React.lazy(() => import('./Chart'));


With Suspense fallback:
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}>



Impact:

*   Dashboard TTI ↓ 1.2s

3. Vendor Library Optimization
-------------------------------
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Analyzed bundles with:

npx source-map-explorer build/static/js/*.js


Found moment.js (300KB) being fully imported! Fixed with:
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import moment from 'moment'; // ❌ 300KB
↓
import format from 'moment/locale/format'; // ✅ 5KB


Impact:

*   Vendor bundle ↓ 65%

The Results
-----------

After 2 weeks of optimization:
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After:

  • main.chunk.js 800KB (↓42%)
  • vendor.chunk.js 150KB (↓62%)



Real-World Impact:
🚀 Faster loads: TTI from 3.5s → 1.9s
📈 Higher engagement: Bounce rate ↓ to 12%
🔼 SEO boost: Lighthouse score → 89/100

Key Takeaways
-------------

1.  “Lazy load everything above the fold”
2.  Audit vendor libs — 60% are often unused
3.  Suspense + Error Boundaries = Seamless UX

What’s your biggest bundle size headache? Drop a comment!

[🔗 _Read my last post: Fixing React Input Lag with useTransition_](https://medium.com/@priyenmehta27/fixing-react-input-lag-how-usetransition-usedeferredvalue-saved-our-ux-666d9514219b)

!! Thanks For Reading !!

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