React 19 is Slow? How to Fix Interaction to Next Paint (INP) Regressions
React 19 was promised as a performance leap. Yet, engineering teams are reporting significant spikes in INP and rendering delays. Here is why your migration might be slowing down your app.
The transition to React 19 has been anything but smooth for high-traffic applications. While the New Architecture promises better concurrent rendering, several breaking changes in internal scheduling have introduced subtle regressions that kill your Google Core Web Vitals.
1. The Death of Parallel Suspense (Issue #29898)
One of the most controversial changes in React 19 is how it handles sibling components within a Suspense boundary. In React 18, siblings would render in parallel even if one suspended. In React 19, this behavior has changed to avoid "partially rendered" states.
// GitHub Issue #29898: Disabling prerendering siblings of suspended components
The result? Massive waterfalls. If you have three components in a Suspense boundary fetching data, they now often wait for each other, turning a 500ms parallel fetch into a 1.5s sequential slog.
2. The `use` Hook Overhead (Issue #29855)
The new use() hook is the recommended way to consume promises in render. However, early adopters have noticed that in complex trees, use() can be significantly slower than the legacy pattern of throwing a Promise.
Community benchmarks (see GitHub Issue #29855) show that under heavy user interaction, the reconciliation overhead of tracking these promises can lead to INP values exceeding 1000ms.
3. The 300ms "Ghost" Delay
React 19 introduces a default FALLBACK_THROTTLE_MS of 300ms. This is intended to prevent "flickering" fallbacks. However, if your interaction completes in 50ms, React might still hold the fallback for the full 300ms duration, making the app feel sluggish and unresponsive to the end-user.
Audit Your Migration
Don't guess why your INP is failing. Our React 19 Migration Auditor scans your component tree for Suspense waterfalls and use() hook anti-patterns automatically.
How to Fix It Today
- Hoist Data Fetching: Move your data fetching to Server Components or route loaders to bypass client-side Suspense waterfalls.
- Debounce Interactions: Use
useTransitionmore aggressively to prevent heavy renders from blocking the main thread. - Profile the Interaction: Use the React DevTools "Interaction" tab to identify which component is holding the main thread during INP events.