Streaming and partial rendering in NextJS - Performance & Optimization

Performance: Streaming and partial rendering

HIGH IMPACT

This affects how quickly the user sees meaningful content by sending parts of the page as soon as they are ready.

Rendering a large page with multiple components

NextJS

import { Suspense } from 'react';

export default async function Page() {
  const dataPromise = fetchDataAsync();
  return (
    <>
      <Header />
      <Suspense fallback={<Loading />}>
        <MainComponent data={await dataPromise} />
      </Suspense>
    </>
  );
}

Streams header immediately and renders main content as soon as data arrives, reducing wait time.

📈 Performance GainReduces LCP by showing partial content early, improving perceived load speed.

Rendering a large page with multiple components

NextJS

export default function Page() {
  const data = fetchDataSync();
  return <MainComponent data={data} />;
}

The entire page waits for all data before sending anything to the browser, delaying first content paint.

📉 Performance CostBlocks rendering until all data is ready, increasing LCP by several seconds on slow networks.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full synchronous render	Single large DOM update	1 large reflow	High paint cost due to delayed content	[X] Bad
Streaming with partial rendering	Multiple smaller DOM updates	Multiple small reflows	Lower paint cost with faster visible content	[OK] Good

Rendering Pipeline

Streaming sends HTML chunks progressively to the browser, allowing it to start parsing and painting parts of the page before the full content is ready.

→HTML Parsing

→Style Calculation

→Layout

→Paint

⚠️ BottleneckWaiting for full data before sending HTML delays HTML Parsing and Paint stages.

Core Web Vital Affected

LCP

This affects how quickly the user sees meaningful content by sending parts of the page as soon as they are ready.

Optimization Tips

1Send HTML chunks as soon as parts of the page are ready to reduce wait time.

2Use React Suspense or Next.js streaming features to enable partial rendering.

3Avoid blocking the entire page render on slow data fetching to improve LCP.

Performance Quiz - 3 Questions

What is the main performance benefit of streaming and partial rendering in Next.js?

A Smaller JavaScript bundle size

B Reduced server CPU usage

C Faster display of initial content to the user

D Improved SEO ranking

DevTools: Performance

How to check: Record a page load and look for the time when the first meaningful paint occurs and when the full content is loaded.

What to look for: Check if content appears progressively (multiple paint events) rather than all at once, indicating streaming is working.

Practice

1. What is the main benefit of streaming in Next.js when rendering pages?

easy

A. It caches the whole page on the client side before showing anything.

B. It delays sending the entire page until all data is loaded.

C. It disables server-side rendering completely.

D. It sends parts of the page to the browser as soon as they are ready.

Solution

1. Step 1: Understand streaming concept in Next.js

   Streaming means sending parts of the page to the browser immediately when they are ready, not waiting for the whole page.

2. Step 2: Compare options with streaming behavior

   It sends parts of the page to the browser as soon as they are ready. matches this behavior exactly, while others describe delaying or caching, which are not streaming.

3. Final Answer:

   It sends parts of the page to the browser as soon as they are ready. -> Option D

4. Quick Check:

   Streaming = send parts early [OK]

Hint: Streaming means show parts early, not wait for all [OK]

Common Mistakes:

• Thinking streaming waits for full page
• Confusing streaming with client caching
• Assuming streaming disables server rendering

2. Which of the following is the correct way to use Suspense for partial rendering in Next.js?

easy

A. } />

B. } />

C. }>

D. }>

Solution

1. Step 1: Recall Suspense usage pattern

   Suspense wraps the async component and uses the fallback prop to show a loading state while waiting.

2. Step 2: Match correct syntax

   <Suspense fallback=<Loading />><AsyncComponent /></Suspense> correctly wraps AsyncComponent inside Suspense with fallback. Others misuse fallback or component placement.

3. Final Answer:

   <Suspense fallback=<Loading />><AsyncComponent /></Suspense> -> Option C

4. Quick Check:

   Suspense wraps async with fallback [OK]

Hint: Suspense wraps async component with fallback prop [OK]

Common Mistakes:

• Placing fallback inside async component
• Not wrapping async component with Suspense
• Using fallback incorrectly as a child

3. Given this Next.js server component code using streaming:

export default async function Page() {
  const user = await fetchUser();
  return (
    <>
      
Welcome {user.name}
      Loading posts...
}>
        
      
    </>
  );
}

What will the user see first in the browser?

medium

A. Only 'Loading posts...' until all data loads.

B. The heading with user name, then 'Loading posts...' while posts load.

C. The full page with heading and posts at once.

D. An empty page until fetchUser finishes.

Solution

1. Step 1: Analyze async data fetching and rendering

   The user data is awaited before rendering, so the heading with user name shows immediately.

2. Step 2: Understand Suspense fallback behavior

   The Posts component is wrapped in Suspense with a fallback, so 'Loading posts...' shows while posts load.

3. Final Answer:

   The heading with user name, then 'Loading posts...' while posts load. -> Option B

4. Quick Check:

   Await user shows heading first, Suspense fallback next [OK]

Hint: Awaited data shows first; Suspense fallback shows during async child load [OK]

Common Mistakes:

• Thinking Suspense fallback shows before awaited user data
• Assuming full page waits for all data
• Ignoring Suspense fallback usage

4. Identify the error in this Next.js streaming code snippet:

export default async function Page() {
  return (
    Loading...
}>
      
    
  );
}

function AsyncData() {
  const data = fetchData();
  return <p>{data.message}</p>;
}

medium

A. AsyncData is not marked async and fetchData is not awaited.

B. Suspense cannot wrap components in Next.js.

C. fetchData should be called inside Page, not AsyncData.

D. The fallback prop must be a string, not JSX.

Solution

1. Step 1: Check AsyncData function for async usage

   AsyncData calls fetchData() but does not await it or mark function async, causing a promise to be rendered.

2. Step 2: Confirm Suspense usage and fallback validity

   Suspense usage is correct, and fallback can be JSX, so no error there.

3. Final Answer:

   AsyncData is not marked async and fetchData is not awaited. -> Option A

4. Quick Check:

   Async function must await async calls [OK]

Hint: Async components must await async calls inside [OK]

Common Mistakes:

• Not marking component async when using await
• Rendering unresolved promises
• Misunderstanding Suspense fallback types

5. You want to stream a page showing user info and their posts. User info loads fast, posts load slowly. How do you combine streaming and partial rendering to show user info immediately and posts with a loading fallback?

hard

A. Make Page async, await user data, render user info immediately, wrap Posts in Suspense with fallback.

B. Render user info and posts inside one Suspense with fallback for both.

C. Fetch posts first, then user info, and render all at once without Suspense.

D. Use client-side fetching for posts and server-side for user info without Suspense.

Solution

1. Step 1: Await fast user data in Page component

   Mark Page async and await user data so user info renders immediately.

2. Step 2: Wrap slow posts component in Suspense with fallback

   Wrap Posts in Suspense with a loading fallback to show partial rendering while posts load.

3. Final Answer:

   Make Page async, await user data, render user info immediately, wrap Posts in Suspense with fallback. -> Option A

4. Quick Check:

   Await fast data, Suspense slow data [OK]

Hint: Await fast data, Suspense slow parts with fallback [OK]

Common Mistakes:

• Wrapping all content in one Suspense losing immediate render
• Not awaiting fast data before rendering
• Using client fetching without Suspense for server components