Optimistic updates pattern in NextJS - Performance & Optimization

Performance: Optimistic updates pattern

MEDIUM IMPACT

This pattern affects interaction responsiveness and perceived speed by updating UI before server confirmation.

Updating UI immediately after user action without waiting for server response

NextJS

function handleClick() {
  setData(prev => ({ ...prev, updated: true })); // update UI immediately
  fetch('/api/update', { method: 'POST' }).catch(() => {
    setData(prev => ({ ...prev, updated: false })); // rollback on error
  });
}

<button onClick={handleClick}>Update</button>

UI updates instantly on user action, improving perceived speed and responsiveness.

📈 Performance GainReduces input delay to near zero, improving INP metric significantly.

Updating UI immediately after user action without waiting for server response

NextJS

async function handleClick() {
  const response = await fetch('/api/update', { method: 'POST' });
  if (response.ok) {
    setData(await response.json());
  }
}

<button onClick={handleClick}>Update</button>

UI waits for server response before updating, causing input delay and poor responsiveness.

📉 Performance CostBlocks interaction feedback for 200-500ms depending on network, increasing INP.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Waiting for server response before UI update	Multiple DOM updates after async call	Triggers 1 reflow per update delay	Higher paint cost due to delayed changes	[X] Bad
Optimistic UI update before server response	Single immediate DOM update	Single reflow triggered instantly	Lower paint cost with smooth interaction	[OK] Good

Rendering Pipeline

Optimistic updates modify the UI state immediately, triggering style recalculation and layout before server response arrives.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout and Paint stages due to immediate DOM updates

Core Web Vital Affected

INP

This pattern affects interaction responsiveness and perceived speed by updating UI before server confirmation.

Optimization Tips

1Update UI state immediately on user input to improve responsiveness.

2Rollback UI changes if server response indicates failure.

3Batch DOM updates to minimize layout thrashing and paint cost.

Performance Quiz - 3 Questions

What is the main performance benefit of using optimistic updates in a Next.js app?

A Improves input responsiveness by updating UI immediately

B Reduces bundle size by removing server calls

C Eliminates all layout recalculations

D Prevents any paint operations on user input

DevTools: Performance

How to check: Record a session while interacting with the UI; look for long tasks or delays between input and next paint.

What to look for: Short input-to-next-paint times indicate good optimistic updates; long delays show poor responsiveness.