How does JavaScript affect load time?

JavaScript is one of the most common performance brakes, because it is more expensive than its file size suggests. Every script must be downloaded, parsed, compiled and executed before the page is fully usable, and parsing blocks the main thread while it happens. Synchronously included scripts additionally delay rendering. Particularly wasteful is unused code that is loaded but never needed, yet still costs bandwidth and processing time.

We tackle this systematically: coverage analysis to identify dead code, code splitting so each page only loads its own scripts, tree shaking to remove unused functions, and deferring non-critical scripts until after the first render. This improves not only LCP but also INP, because fewer long tasks block interactivity. Details on the frontend optimization page.