CSS-in-JS considerations in NextJS - Performance & Optimization

Performance: CSS-in-JS considerations

MEDIUM IMPACT

This affects page load speed and rendering performance by how styles are generated and applied in the browser.

Applying styles in a Next.js app using CSS-in-JS

NextJS

import { css } from '@emotion/react';

const buttonStyle = css`
  color: red;
  font-size: 1rem;
`;

export default function Page() {
  return <button css={buttonStyle}>Click me</button>;
}

Using Emotion with server-side rendering extracts styles at build time, reducing runtime style generation and improving load speed.

📈 Performance GainReduces blocking time by 50-100ms; smaller runtime JS bundle

Applying styles in a Next.js app using CSS-in-JS

NextJS

import styled from 'styled-components';

const Button = styled.button`
  color: red;
  font-size: 1rem;
`;

export default function Page() {
  return <Button>Click me</Button>;
}

Styles are generated at runtime on the client, causing a flash of unstyled content and delaying Largest Contentful Paint.

📉 Performance CostBlocks rendering for 100-200ms on slow devices; adds ~10-20kb to JS bundle

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Runtime CSS-in-JS (styled-components)	Low	1 per style injection	Medium due to delayed styles	[X] Bad
Static CSS extraction (Emotion SSR)	Low	Single reflow	Low paint cost	[OK] Good

Rendering Pipeline

CSS-in-JS styles are generated either at build time or runtime. Runtime generation delays style calculation and layout, causing slower paint.

→Style Calculation

→Layout

→Paint

⚠️ BottleneckStyle Calculation due to runtime style injection

Core Web Vital Affected

LCP

This affects page load speed and rendering performance by how styles are generated and applied in the browser.

Optimization Tips

1Avoid generating CSS styles at runtime to reduce blocking time.

2Use server-side rendering or static extraction for CSS-in-JS styles.

3Monitor Largest Contentful Paint to detect style injection delays.

Performance Quiz - 3 Questions

What is a main performance drawback of runtime CSS-in-JS in Next.js?

A It delays style calculation causing slower Largest Contentful Paint

B It increases server CPU usage during build

C It reduces JavaScript bundle size

D It improves caching of styles

DevTools: Performance

How to check: Record page load and look for long tasks related to style recalculation and layout shifts caused by style injection.

What to look for: Look for delayed Largest Contentful Paint and style recalculation events indicating runtime CSS generation.