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If your React app feels slow, first identify whether the delay comes from rendering, JavaScript loading or execution, or the network. Those problems can feel alike to users, but each calls for a different fix. Treat the three causes below as troubleshooting paths—not a universal ranking—and measure before changing code.
How to diagnose a slow React app
Reproduce one slow initial load or interaction in a production-like build. Note what you did and how long the wait felt, then record a browser Performance trace. Compare network requests, JavaScript execution, event-loop activity, and React events rather than assuming a render is responsible. React Performance tracks can show React-specific activity alongside browser work: React Performance tracks.
- Reproduce one delay. Choose a specific case, such as opening a route or typing into a search box, and use the same action for each comparison.
- Record a browser trace. Look for time spent waiting on requests, executing JavaScript, or processing event-loop work as well as React activity.
- Inspect React commits. Use the React Developer Tools Profiler or React’s
<Profiler>to locate slow commits and subtrees. Profiling adds overhead; ordinary production builds disable the relevant profiling instrumentation by default. See React Developer Tools and <Profiler>. - Change one thing that matches the evidence. Then repeat the same measurement and confirm that the user-facing delay improved.
Without a before-and-after comparison, a code change is only a plausible fix—not a demonstrated one.
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A parent update can cause its children to render again. That is often harmless. It becomes a performance issue when a component or subtree does enough work for repeated rendering to affect the interaction, or when an expensive calculation runs again even though its inputs have not changed.
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Find the work that is actually costly
Use the Profiler to find the slow commit or subtree, then inspect the components and calculations involved. Keep state close to the UI that needs it, and avoid unnecessary work or effects that trigger extra updates. React’s guidance on when you might not need an Effect can help identify calculations or state updates that do not need to be managed through an effect.
Use memoization selectively
useMemo can cache a calculation between renders when its dependencies have not changed. memo can let a component skip some renders when its props are unchanged. Neither is a guarantee, and both add complexity if applied without a measured need. Check dependencies carefully and use them for a demonstrated bottleneck with stable inputs. See React’s documentation for useMemo and memo.
Check whether your project uses React Compiler before adding manual memoization everywhere: it can automatically memoize many components and values.
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2. Too much JavaScript loads before the app is usable
If the initial screen waits for code needed only by other routes or features, the delay may come from the amount of JavaScript sent and processed up front—not from React repeatedly rendering. React’s guide to building a React app from scratch explains that growing code and dependencies can slow loading when the entire app must be sent before use.
Split code that is not needed immediately
Inspect the production bundle and identify large routes or features that users do not need for the first screen. React’s lazy lets you defer a component’s code until its first render; use it with Suspense to show a loading state while that code loads. See lazy.
Split selectively, keep the initial view useful, and measure the result. Code splitting can sometimes make loading slower, so adding a lazy boundary is not proof that the experience improved.
3. Network delays or browser work look like React slowness
A slow experience can come from sequential network requests, JavaScript execution, or other event-loop activity even when React rendering is not the bottleneck. The browser trace and network panel help distinguish those delays from React work.
Look for request waterfalls
If one request cannot start until another finishes, the user may wait through a chain of network delays. React’s app-building guide cautions that fetching data directly in components can create slower-loading network waterfalls. Where your application architecture allows, consider loading data through route loaders or on the server instead.
Best Value
Keep input responsive while a slow view catches up
For a large list or chart that updates on every keystroke, useDeferredValue can let the input respond while the slower view catches up. React’s example requires the slow component to be memoized. Deferring the view changes when the work happens; it does not make the calculation itself cheaper. See useDeferredValue.
Choose the fix that matches the delay
| Evidence from measurement | Fix to investigate | Trade-off to check |
|---|---|---|
| A slow React commit or repeated expensive calculation | Reduce unnecessary work; consider memo or useMemo for the measured bottleneck. |
Memoization adds complexity and only helps when its conditions are met. |
| A long initial wait associated with code needed only later | Split noncritical routes or features and load them on demand. | Splitting can add loading delays; preserve a usable initial view. |
| Time dominated by sequential requests or browser JavaScript work | Address data-loading timing or the execution work shown in the trace. | Deferring a view can improve responsiveness without reducing its total work. |
After making one targeted change, repeat the same interaction and compare the trace and the felt delay. Keep the change only if it improves the experience you were investigating.
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