After enabling React Compiler in a React 19 dashboard, our team removed about 60% of its manual useMemo and useCallback calls across three small pull requests. The change cut code and dependency-array maintenance, but it did not make every screen faster: already well-memoized areas stayed within measurement noise, while previously over-rendering screens showed the clearest improvement.
What changed in our React 19 dashboard
The dashboard combines a large virtualized data table, several charts, and a draggable timeline scrubber. Over time, defensive optimization habits and code-review conventions had left 214 useMemo and useCallback calls in the codebase.
We enabled React Compiler gradually, first in annotation mode and then in default mode. For components we were not ready to trust, we used the "use no memo" directive to opt out. Before removing existing memoization, we ran the React Compiler healthcheck and enabled compiler-aware lint rules from eslint-plugin-react-hooks. Lint findings showed which components could not be compiled and gave us a practical migration backlog.
What we removed—and what the compiler took over
Across three feature-scoped pull requests, we removed about 60% of the manual memoization: roughly 214 calls became about 80. Each remaining memo had a documented reason to stay.
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One representative simplification was a filtered, sorted list. Without manual memoization, its code no longer needed dependency arrays or the associated stale-closure risk, and we could remove layers of wrappers that had accumulated around the calculation. The compiler also handled calculations that were awkward to memoize by hand, including a summary calculation after a possible early return.
These counts and the removal rate are observations from Faisal Mujtaba’s single application in 2026, not a target every React project should try to match.
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What our measurements showed
Before and after each pull request, we replayed the same scripted interaction and compared commit counts and render durations in React DevTools Profiler. We also compared real-user Interaction to Next Paint (INP) over one week before and one week after, and checked bundle size because compiler output adds code.
| Area or measure | What we observed | What that means |
|---|---|---|
| Already well-memoized areas | Commit counts and render durations stayed within measurement noise. | Compiler adoption did not produce a measurable speedup there. |
| Settings panel and several screens | These had been re-rendering entire subtrees on every keystroke. INP improved noticeably on mid-range Android devices. | The clearest gains came where our prior render behavior left room for improvement. |
| Main bundle | It grew slightly—described in our 2026 report as a couple of percent. | Compiler output has a size cost worth checking in the application, rather than assuming it is negligible. |
Our summary was: “no speedup where we’d been careful, a real one where we hadn’t, and a lot less code overall.” These results came from one app and one set of measurements; they are not independent benchmarks or a guarantee of similar gains elsewhere.
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Where compilation stopped helping
A component mutated an input array
In staging, the compiler silently skipped a component that sorted a scores prop in place. We fixed it by copying the array before sorting. Treat compiler skips as a signal to inspect the component’s assumptions and data handling, not as proof that the component is already optimized.
A legacy chart wrapper broke prop identity
A legacy chart wrapper passed a newly created options object to a compiled child on every render. Since the prop identity changed before the compiled code, we restored manual useMemo at that boundary. Compiler guarantees do not extend across uncompiled parents or third-party components that rely on reference equality.
Rank #4
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Why the timeline scrubber still stuttered
The scrubber’s problem was not a missing memo. Dragging produced more than 60 state updates per second as pointermove events flowed through React. We moved drag updates to a CSS variable updated with requestAnimationFrame, then told React about the result only when the drag ended. That reduced the work React had to process; memoization alone could not make continuous rendering free.
A safer sequence for removing manual memoization
- Enable the compiler and run the healthcheck. Do this before deleting existing
useMemooruseCallbackcalls, so you can see the compiler’s coverage and problems first. - Fix lint violations. Use compiler-aware
eslint-plugin-react-hooksfindings as a backlog of components to review and repair. - Roll out gradually. Start in annotation mode, or use
"use no memo"to opt out components that are not ready for compilation. - Remove memoization in small, feature-scoped pull requests. Smaller changes make it easier to identify which deletion caused a regression.
- Profile before and after each change. Compare the same interaction and inspect commit counts and render durations; check user-facing interaction measurements and bundle size where relevant.
- Keep manual memoization at boundaries that need it. Retain it where an uncompiled parent or third-party component depends on stable reference identity.
- Use browser primitives for high-frequency input. If pointer, scroll, or drag events overwhelm React state updates, consider refs, CSS variables, and
requestAnimationFramerather than trying to solve event frequency with memoization.
When should you delete useMemo and useCallback?
Delete them incrementally when the compiler covers the component and profiling shows that the change is safe. Keep a manual memo when a specific boundary needs stable reference identity, when the compiler cannot handle the component, or when measured behavior regresses without it. If the bottleneck is high-frequency input, reduce the work sent through React instead of adding memoization around every update.
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- GIFT QUALITY NOTEBOOKS: Moleskine planners, journals and notebooks come in hardcover or softcover and colors like black, red, blue, green and brown. The binding and cover have a durable finish, designed for daily journaling, writing and sketching.
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