CSS Modules in NextJS - Performance & Optimization

Performance: CSS Modules

MEDIUM IMPACT

CSS Modules affect how styles are loaded and scoped, impacting page load speed and style recalculation.

Applying styles scoped to components without global conflicts

NextJS

import styles from './Button.module.css';

function Button() {
  return <button className={styles.primary}>Click me</button>;
}

CSS Modules load only styles used by the component, reducing CSS size and avoiding conflicts.

📈 Performance GainSaves 30-70kb in CSS bundle, faster LCP due to smaller CSS

Applying styles scoped to components without global conflicts

NextJS

import './styles.css';

function Button() {
  return <button className="btn primary">Click me</button>;
}

Global CSS loads all styles on every page, increasing CSS size and causing potential style conflicts.

📉 Performance CostAdds 50-100kb to CSS bundle, blocks rendering until CSS loads

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Global CSS import	Low (styles applied globally)	Low (styles trigger once)	Medium (large CSS size)	[!OK]
CSS Modules import	Low (scoped styles)	Low (minimal style recalculation)	Low (smaller CSS size)	[OK] Good

Rendering Pipeline

CSS Modules generate scoped CSS classes that the browser applies during style calculation and layout, reducing style recalculation and repaint.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation

Core Web Vital Affected

LCP

CSS Modules affect how styles are loaded and scoped, impacting page load speed and style recalculation.

Optimization Tips

1Use CSS Modules to scope styles locally and reduce global CSS size.

2Smaller CSS bundles improve Largest Contentful Paint (LCP).

3Avoid loading large global CSS files that block rendering.

Performance Quiz - 3 Questions

How do CSS Modules improve page load performance compared to global CSS?

A By using inline styles instead of CSS files

B By loading only component-specific styles, reducing CSS bundle size

C By loading all styles globally to cache them faster

D By delaying CSS loading until user interaction

DevTools: Network

How to check: Open DevTools, go to Network tab, filter by CSS, and compare CSS file sizes loaded with global CSS vs CSS Modules.

What to look for: Look for smaller CSS file sizes and fewer style recalculations in Performance tab for CSS Modules.