I Cut a React App's Bundle from 450KB to 120KB. Here's How.

Every kilobyte of JavaScript you ship gets downloaded, parsed, compiled, and executed. On a budget Android phone over 4G, a 500KB bundle takes 2-3 seconds — and your users feel every millisecond. This guide walks through the exact steps I use, with real package size data from PkgPulse . The biggest single win? Swapping moment.js for day.js saves 70KB in one line change. Originally published on pkgpulse.com Why Bundle Size Matters More Than Ever When a browser downloads your JavaScript, it doesn't just download it. It has to: Download — connection-dependent (3G: ~400KB/s, 4G: ~4MB/s) Parse — read and validate JavaScript syntax Compile — V8/SpiderMonkey compiles to machine code Execute — initialization, event listeners, framework bootstrap Steps 2-4 are CPU-bound. A 500KB bundle that loads in 200ms on your MacBook Pro might take 2-3 seconds on a mid-range phone. Your users feel that. The SEO Impact Google's Core Web Vitals measure three things, and bundle size affects all of them: LCP (L