Synthetic events in React - Performance & Optimization

Performance: Synthetic events

MEDIUM IMPACT

Synthetic events affect how user interactions are handled and can impact input responsiveness and event processing speed.

Handling user clicks efficiently in React

React

function Button() {
  return <button onClick={() => console.log('Clicked')}>Click me</button>;
}

Avoids unnecessary event persistence, allowing React to reuse event objects and optimize memory.

📈 Performance GainReduces memory overhead and keeps event handling fast and responsive.

Handling user clicks efficiently in React

React

function Button() {
  return <button onClick={(e) => { e.persist(); console.log('Clicked'); }}>Click me</button>;
}

Calling e.persist() disables React's event pooling, increasing memory usage and delaying garbage collection.

📉 Performance CostIncreases memory usage and can cause minor delays in event processing under heavy interaction.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Using React synthetic events normally	No extra DOM ops	0	0	[OK] Good
Calling e.persist() on synthetic events	No extra DOM ops	0	0	[!] OK - higher memory use
Using native DOM events directly in React	No extra DOM ops	0	0	[!] OK - bypasses React pooling
Heavy event handler logic inside synthetic events	No extra DOM ops	0	0	[X] Bad - slows input responsiveness

Rendering Pipeline

Synthetic events are created by React to normalize events across browsers. When a user interacts, React captures the native event, wraps it in a synthetic event, and passes it to handlers. This adds a small step before the event reaches your code.

→Event Handling

→JavaScript Execution

⚠️ BottleneckJavaScript Execution due to event object creation and pooling management

Core Web Vital Affected

INP

Synthetic events affect how user interactions are handled and can impact input responsiveness and event processing speed.

Optimization Tips

1Avoid calling e.persist() unless you need to keep the event beyond the handler.

2Keep event handlers simple and fast to improve input responsiveness.

3Use React synthetic events to benefit from cross-browser normalization with minimal overhead.

Performance Quiz - 3 Questions

What is the main performance cost of React synthetic events compared to native events?

A They add a small JavaScript processing step to normalize events.

B They cause extra DOM reflows on every event.

C They increase CSS selector complexity.

D They block rendering until the event finishes.

DevTools: Performance

How to check: Record a performance profile while interacting with the component. Look for event handler execution time and memory usage.

What to look for: Check if event handlers are quick and if there is excessive memory retained due to event persistence.