Font optimization with next/font in NextJS - Performance & Optimization

Performance: Font optimization with next/font

HIGH IMPACT

This affects page load speed by optimizing font delivery and rendering, reducing blocking time and layout shifts.

Loading custom fonts in a Next.js app

NextJS

import { Inter } from 'next/font/google';
const inter = Inter({ subsets: ['latin'], variable: '--font-inter' });

export default function App({ Component, pageProps }) {
  return <main className={inter.variable}><Component {...pageProps} /></main>;
}

next/font automatically optimizes font loading with preloading, subsets, and no layout shifts.

📈 Performance GainReduces LCP by preloading fonts, eliminates CLS from font swapping

Loading custom fonts in a Next.js app

NextJS

import '../styles/globals.css';
// In globals.css
@font-face {
  font-family: 'CustomFont';
  src: url('/fonts/customfont.woff2') format('woff2');
  font-display: swap;
}

// Use font-family: 'CustomFont' in CSS

This approach loads fonts via CSS, causing render-blocking and potential layout shifts as fonts load asynchronously.

📉 Performance CostBlocks rendering until font is downloaded, causes CLS due to font swap

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
CSS @font-face with font-display: swap	Minimal DOM impact	Triggers reflow on font swap	Paint cost on font swap	[!] OK
next/font with automatic preload and subset	Minimal DOM impact	Single reflow during initial layout	Reduced paint cost	[OK] Good

Rendering Pipeline

Font loading affects the critical rendering path by blocking text rendering until fonts are ready or fallback fonts are used. next/font preloads fonts and injects CSS to minimize layout shifts and reflows.

→Critical Rendering Path

→Style Calculation

→Layout

→Paint

⚠️ BottleneckCritical Rendering Path blocking due to font download and layout shifts during font swap

Core Web Vital Affected

LCP, CLS

This affects page load speed by optimizing font delivery and rendering, reducing blocking time and layout shifts.

Optimization Tips

1Always preload fonts to reduce blocking time on text rendering.

2Use font-display strategies to avoid layout shifts caused by font swapping.

3Leverage next/font to automatically optimize font subsets and loading.

Performance Quiz - 3 Questions

What is the main performance benefit of using next/font in Next.js?

A Automatic font preloading and reduced layout shifts

B Larger font file sizes for better quality

C Delays font loading until after page load

D Disables font fallback to speed up rendering

DevTools: Performance

How to check: Record a page load in the Performance panel, then look for font download timing and layout shifts in the summary.

What to look for: Check for reduced blocking time on font download and minimal layout shifts (CLS) during text rendering.