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A free scan shows the junk files, broken settings and background clutter dragging Windows down - then fixes them in one click.Free scan · Windows 10 & 11To improve React performance, first reproduce a specific slow interaction and profile the work it triggers. Fix unnecessary state updates and rendering before adding memoization; then make a focused change and measure the same interaction again. A component re-render is not automatically a problem—optimize when users can feel the slowdown or profiling shows repeated work worth addressing.
Start with a specific slowdown
Name the interaction that feels slow and what you expect to happen: filtering a list, typing into a field, changing routes, or updating a large view. Keeping the question concrete helps distinguish a real bottleneck from harmless rendering. React notes that memoization is usually unnecessary for coarse interactions that do not feel slow.
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Test the same action consistently before and after a change. If development runs on a much faster machine than users’ devices, CPU throttling can help make the difference more apparent. For timing that reflects actual use, React recommends profiling a production build on a device like the devices your users have.
Find the repeated work with a profiler
Use the Profiler tab in React Developer Tools to inspect the component tree while reproducing the interaction. Look for components that render repeatedly and determine whether they are doing expensive work or simply updating as expected.
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For programmatic measurements, wrap the relevant subtree in React’s <Profiler> and provide an id and onRender callback:
<Profiler id="SearchResults" onRender={handleRender}>
<SearchResults results={results} />
</Profiler>The callback’s actualDuration measures rendering work for the current update. baseDuration estimates how long the subtree would take to render without optimizations. These are render measurements, not universal scores for the time a user waits. The Profiler adds overhead and is disabled in the standard production build; production profiling requires React’s special profiling-enabled build.
Remove avoidable update work first
Before adding caches, check whether the app is triggering work it does not need. React identifies unnecessary Effects that update state and cause chains of renders as a common source of performance problems. Keep rendering pure and correct rendering bugs rather than trying to hide them with memoization.
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- Review Effects that set state. If an Effect triggers another render, check whether the derived value can be calculated during rendering or whether the update is needed at all.
- Pass JSX as children when appropriate. A wrapper that receives its content as children can update without requiring the wrapper’s parent to recreate that content.
- Keep render logic pure. Rendering should not depend on side effects or mutate values in ways that make repeated renders behave differently.
Choose memoization for the measured cause
Memoization can skip repeated calculations or rendering when inputs have not changed. It is useful only when dependencies and props remain stable, and it adds complexity; a newly created object or function can defeat it.
| Tool | What it can cache or skip | When it may help |
|---|---|---|
useMemo |
A calculation result | A noticeably slow calculation whose dependencies rarely change, a stable value passed to a memoized child, or a value used as another Hook’s dependency. |
useCallback |
A function reference | A callback passed to a memoized child when its dependencies remain unchanged. |
memo |
A component render when props are unchanged | A component often receives the same props and its rendering work is expensive. |
useMemo: cache a calculation result
useMemo reuses a calculated value while its dependencies remain unchanged. Use it when profiling points to repeated, expensive calculation—not simply because a value exists. A fresh object or function in the dependency list changes identity and can cause the calculation to run again.
useCallback: keep a function reference stable
useCallback caches a function reference while its dependencies remain unchanged. Its clearest performance use is a callback passed to a memoized child: a new function reference on every parent render can make the child’s props appear changed.
memo: skip rendering when props are unchanged
memo can let a component skip rendering when its props are unchanged. It is most useful when the component receives the same props often and its rendering is costly. If a parent supplies a newly created object or function as a prop each time, the props are not unchanged, so the optimization may not help.
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React’s documentation cautions that useMemo is a performance optimization, not a semantic guarantee, and says to rely on memo only as a performance optimization. The app must remain correct if React does not reuse a cached value or skip a render.
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Account for React Compiler
React Compiler is a build-time tool that can automatically optimize React code by applying memoization based on its analysis and heuristics. React’s current guidance recommends relying on the compiler for memoization in most new code, while using Hooks when precise control is useful.
If a project already has manual memoization, React advises leaving it in place or removing it only after careful testing; changing it can affect compiler output. Compiler optimization also does not make code that relies on referential equality for correctness safe. React’s compiler debugging guidance identifies that kind of assumption as a potential problem.
Compare the same interaction after the change
- Reproduce the original slowdown and note which component or calculation is doing repeated work.
- Make one focused change, such as removing an unnecessary state-update chain or stabilizing a value used by an expensive memoized child.
- Repeat the same interaction under comparable conditions and inspect the relevant profile again.
- Confirm that the UI remains correct, including when inputs change and cached values are not reused.
Keep a change only if the measurement and user-visible behavior support it. React’s documentation does not establish that a particular compiler setting or memoization change will make a given app faster; results depend on the code, workload, device, and measurement context.
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