Optimizing JavaScript & CSS

JavaScript and CSS play a major role in how your website looks and behaves. But when these files are too large, poorly organized, or loaded at the wrong time, they can slow down your website and hurt your Core Web Vitals—especially INP and LCP.

This guide explains how to optimize JavaScript and CSS so your website loads faster, responds more quickly, and provides a smoother experience for visitors.

Why JavaScript & CSS Affect Website Performance

Browsers must download, parse, and execute JavaScript and CSS before your page can fully load. Heavy or inefficient code can cause:

• Slow loading times — delaying LCP
• Unresponsive interactions — hurting INP
• Layout shifts — if styles load late
• Long main-thread blocking — making the site feel sluggish

How to Optimize JavaScript

1. Remove Unused JavaScript

Many websites load scripts they don’t actually use—especially from plugins or old features. Removing unused code reduces file size and improves responsiveness.

2. Defer Non-Critical Scripts

Scripts that aren’t needed immediately should be deferred so they load after the main content.

3. Minify JavaScript Files

Minification removes unnecessary characters (like spaces and comments) to reduce file size.

4. Split Large Scripts into Smaller Chunks

Breaking up large JavaScript bundles helps the browser process code more efficiently.

5. Reduce Third-Party Scripts

Tools like chat widgets, analytics, and tracking scripts can significantly slow down your site. Remove anything unnecessary or replace heavy tools with lighter alternatives.

How to Optimize CSS

1. Remove Unused CSS

Many themes and page builders load large CSS files even if only a small portion is used. Removing unused styles improves load times.

2. Minify CSS Files

Minifying CSS reduces file size and speeds up loading.

3. Inline Critical CSS

Critical CSS includes the styles needed to render the above-the-fold content. Inlining it helps the browser display your page faster.

4. Defer Non-Critical CSS

Styles not needed immediately can be loaded later to improve initial rendering.

5. Use Smaller, Modular Stylesheets

Breaking large CSS files into smaller, page-specific files reduces unnecessary loading.

How JavaScript & CSS Affect Core Web Vitals

• LCP: Heavy CSS or render-blocking scripts delay the main content.
• INP: Long JavaScript tasks make the site feel unresponsive.
• CLS: Late-loading styles can cause layout shifts.

How to Measure JavaScript & CSS Performance

You can analyze your code using tools like:

• Google PageSpeed Insights
• Chrome DevTools
• WebPageTest
• GTmetrix

Learn more: Tools for Testing Website Performance

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