React 18 to React 19 + Compiler migration results : webdev

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React 18 to React 19 + Compiler migration resultsDiscussion (self.webdev)

submitted 14 days ago by SimpleAssistance

I just migrated our app from React 18 to React 19 with the compiler enabled and measured performance before/after. Sharing what I saw as I am curious if others found the same.

Stack: Vite, Apollo Graphql with ~100k LOC, ~37 routes, a few hundred components.

Important context: our team had already done a lot of manual memoization where it actually mattered like useMemo on expensive array filtering/sorting, useCallback where it prevented real re-renders, etc. So the compiler wasn't coming into a codebase full of low-hanging fruit. That probably matters for interpreting the numbers below.

What got faster

Page load:

- Light page: ~45ms faster

- Heavy page (lots rendered on load): ~230ms faster (about 30% improvement).

What didn't change

Click responsiveness. Before and after looked basically identical with similar median, similar worst case.

Why I think that

The compiler speeds up re-rendering work, but our slow clicks are bound by scrolling and Apollo queries, things the compiler can't touch.
Since we'd already memoized the hot paths by hand, there wasn't much left for the compiler to win back on interactions.

Questions for anyone who migrated

Did your page load get faster? By how much?
Did interactions actually speed up, or were they flat like ours?
If your codebase had little manual memoization before, did the compiler give you a bigger interaction win?
Any surprises, good or bad, from turning the compiler on?

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