Client-only modules in NextJS - Performance & Optimization

Performance: Client-only modules

MEDIUM IMPACT

This affects page load speed by deferring non-essential JavaScript to client side, improving initial server render and reducing blocking time.

Importing a module that only works in the browser in a Next.js page

NextJS

import dynamic from 'next/dynamic';

const BrowserOnlyComponent = dynamic(() => import('browser-only-module'), { ssr: false });

export default function Page() {
  return <BrowserOnlyComponent />;
}

Defers loading to client only, skipping server render and reducing initial bundle size.

📈 Performance GainReduces server render blocking, saves 50-100kb on initial bundle, improves LCP by 200-400ms

Importing a module that only works in the browser in a Next.js page

NextJS

import BrowserOnlyComponent from 'browser-only-module';

export default function Page() {
  return <BrowserOnlyComponent />;
}

This imports the module during server-side rendering, causing errors or large bundles and blocking initial render.

📉 Performance CostBlocks server render, increases bundle size by 50-100kb, delays LCP by 200-400ms

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Import client-only module normally	N/A on server, large bundle on client	Multiple reflows if module triggers layout	High paint cost due to blocking JS	[X] Bad
Dynamic import with ssr: false	Minimal server DOM, client loads later	Single reflow after load	Lower paint cost, non-blocking	[OK] Good

Rendering Pipeline

Client-only modules are skipped during server rendering, so the server sends minimal HTML. The browser loads these modules asynchronously after initial paint, avoiding blocking style calculation and layout.

→Server Rendering

→JavaScript Parsing

→Layout

→Paint

⚠️ BottleneckServer Rendering and JavaScript Parsing

Core Web Vital Affected

LCP

This affects page load speed by deferring non-essential JavaScript to client side, improving initial server render and reducing blocking time.

Optimization Tips

1Always use dynamic imports with ssr: false for modules that only run in the browser.

2Avoid importing client-only code directly in server-rendered components to prevent blocking.

3Check bundle size and server render time to ensure client-only modules are deferred.

Performance Quiz - 3 Questions

What is the main benefit of using dynamic imports with ssr: false for client-only modules in Next.js?

A It preloads the module before the page loads to avoid delays.

B It bundles the module into the server output for faster server rendering.

C It defers loading to client, reducing server render blocking and improving LCP.

D It disables JavaScript entirely for faster page load.

DevTools: Performance

How to check: Record page load, look for long tasks blocking main thread during initial load and check when client-only module scripts are loaded.

What to look for: Look for reduced blocking time and earlier LCP when using dynamic imports with SSR disabled.