JavaScript Bootup Time

Why It Matters

• Users cannot interact during bootup. While the browser is busy processing JavaScript, buttons do not respond, forms do not submit, and menus do not open. The page looks ready but feels broken.
• Mobile devices suffer the most. A script that boots up in 200ms on a fast laptop might take two or three seconds on a mid-range phone. Most of your audience is probably not on the latest hardware.
• It directly affects Core Web Vitals. High bootup time contributes to poor Total Blocking Time and Interaction to Next Paint scores, both of which measure how responsive your page feels to real users.
• Every kilobyte of JavaScript has a cost. Unlike images, JavaScript must be parsed and executed — not just downloaded. A 200KB script costs far more processing time than a 200KB image.

How to Reduce Bootup Time

1. Remove unused JavaScript. Audit your bundles for code that is imported but never called. Tree-shaking, dead code elimination, and removing unused libraries can significantly shrink what the browser needs to process.
2. Split your code into smaller chunks. Load only the JavaScript needed for the current page. Use dynamic import() to defer loading of features until the user actually needs them.
3. Defer non-critical scripts. Use <script defer> or <script async> for scripts that do not need to run before the page is visible. This lets the browser render the page while scripts download in the background.
4. Minify and compress. Minification removes whitespace, comments, and shortens variable names. Compression (gzip or Brotli) further reduces the file size during transfer. Both together can cut JavaScript size dramatically.
5. Avoid large third-party bundles. Analytics libraries, chat widgets, ad scripts, and social media embeds all add JavaScript. Evaluate whether each one justifies its cost and load them lazily when possible.

Common Mistakes

• Loading everything upfront. Bundling all JavaScript into a single massive file means the browser must process features the user may never use. Split your code and load on demand.
• Testing only on fast devices. A page that feels snappy on your development machine can be painfully slow on a budget phone. Test on real devices or use CPU throttling in your browser's developer tools.
• Ignoring third-party script costs. Third-party scripts often load their own dependencies, creating a chain of downloads and execution. One "small" embed can add hundreds of kilobytes of JavaScript to your page.
• Relying on caching to fix the problem. Caching helps returning visitors, but first-time visitors still pay the full cost. Reducing the amount of JavaScript matters even if your cache strategy is excellent.