"Remove Unused JavaScript" is the most intimidating error in Google PageSpeed Insights. It sounds like you need to delete code, which sounds like breaking your website. Luckily, it's simpler than that.
1. The Traffic Jam Analogy
Imagine a single-lane highway (your website loading process). JavaScript files are like massive trucks. Even if the truck is only carrying one small box (a single feature), the entire highway stops until that truck passes. This is called "Render Blocking."
Defer & Async: These are attributes we add to script tags. They tell the browser, "Park this truck on the side of the road and let the cars (content) pass first. Inspect the truck later."
How to Detect It
Run a SiteSpeed Scan or use Chrome DevTools (Coverage Tab). It will show you exactly which files are being loaded but not used. Often, you are loading a huge library for a slider that is only on the Homepage, but it's being loaded on every single blog post.
2. Treatment Plan
You don't need to write code.
• WordPress: Install WP Rocket or Asset CleanUp. These plugins have checkboxes
to "Defer JavaScript execution" automatically. This is much safer than manually editing the
<script> tags in your HTML header, which can easily break your site.