Discover how code splitting and lazy loading can dramatically improve frontend performance, enhance user experience, and optimise load times for modern Website Designers
Introduction
In today’s digital landscape, speed and responsiveness are critical for retaining users and ensuring a smooth browsing experience. As frontend applications grow more complex and feature-rich, they also become heavier and slower to load. That’s where code splitting and lazy loading come into play.
These modern performance optimisation techniques allow developers to load only what is necessary, significantly improving user experience and website speed.
What is Code Splitting?
Code splitting is the process of breaking down your application’s bundle into smaller, manageable chunks of JavaScript. Instead of delivering one large JavaScript file to the browser, code splitting enables the browser to download only the code required for the current page or component.
Benefits of Code Splitting:
- Faster initial page loads
- Reduces memory footprint
- Better caching strategies
- Optimised loading for route-based applications
What is Lazy Loading?
Lazy loading is a technique that delays the loading of resources until they are actually needed. This can be applied not only to JavaScript files, but also to images, videos, and components.
In the context of frontend development, lazy loading is often used in combination with code splitting. Instead of loading all JavaScript files upfront, you lazy-load individual modules or components on-demand.
Benefits of Lazy Loading:
- Improves page speed
- Reduces Time to Interactive (TTI)
- Better performance on slow networks
- Decreases initial load bundle size
Code Splitting vs Lazy Loading: Are They the Same?
Although they are closely related and often used together, code splitting and lazy loading are not the same:
- Code Splitting: A build-time process that creates multiple JavaScript bundles.
- Lazy Loading: A runtime technique to defer loading of modules until needed.
How Code Splitting Works in Practice
Most JavaScript bundlers and frameworks provide out-of-the-box support for code splitting. Here’s a quick overview of how it works in different ecosystems:
React (with Webpack)
Angular
Vue.js
Lazy Loading Images and Media
Lazy loading isn’t just for JavaScript. You can apply it to images and media assets too using the attribute in HTML.
For more advanced control, you can use the Intersection Observer API to detect when elements enter the viewport and then load them dynamically.
Tools That Help with Code Splitting & Lazy Loading
- Webpack: Supports dynamic and optimises chunks via plugins.
- Vite: Offers fast builds and smart code splitting for modern JS apps.
- Parcel: Zero-config bundler that automatically splits code on dynamic import.
- React Loadable: For older React versions before React.lazy existed.
- Lighthouse: Helps audit performance and shows impact of code splitting.
Best Practices
- Split by Route or Feature
- Avoid Too Many Small Chunks
- Use and Error Boundaries
- Combine with Tree Shaking
- Preload Critical Chunks
- Monitor Bundle Sizes
Real-World Impact
Let’s consider a typical React SPA without any code splitting. On initial load, it might serve a 2MB JavaScript file. On mobile devices or slow networks, this results in long load times, low Lighthouse scores, and poor user experience.
With proper code splitting and lazy loading, that 2MB could be reduced to 300KB initially, with the rest loaded on-demand as users navigate. The result is:
- 70–80% faster initial load
- Improved Google PageSpeed score
- Higher user retention and lower bounce rates
Common Pitfalls to Avoid
- Over-splitting: Creating too many small chunks can lead to inefficient network use.
- Incorrect fallback handling: Always provide a loading UI to manage user expectations.
- Lazy loading above-the-fold content: This can hurt perceived performance.
- Lack of testing: Dynamically imported modules must be thoroughly tested in production scenarios.
Conclusion
Code splitting and lazy loading are not just trendy techniques—they’re essential performance strategies for modern frontend development. By reducing initial load times and serving code on-demand, developers can deliver faster, more responsive, and user-friendly applications.
As mobile users and performance standards continue to rise, applying these optimisations isn't optional—it’s expected. So whether you’re building a new app or refactoring an old one, integrate code splitting and lazy loading into your frontend workflow to stay ahead.
Ready to boost your site’s speed?
Start by analysing your JavaScript bundle and identify opportunities to split and defer code. Use tools like Webpack, Vite, or Parcel to automate the process, and test the results using Lighthouse or Chrome DevTools.
