[core] Regression test is 5 times slower after React 19 upgrade

[DiegoAndai]

From #44672 (comment)

Tests speed increased by x2.4 https://app.circleci.com/insights/github/mui/material-ui/workflows/pipeline/overview?reporting-window=last-60-days after December 1st.

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It seems to be this PR: #44672. The time it takes to run the visual regression tests:

• Before: 4m https://app.circleci.com/pipelines/github/mui/material-ui/144367/workflows/26011c08-72cf-4022-a276-119f1e755c3b/jobs/777254
• After: 19m https://app.circleci.com/pipelines/github/mui/material-ui/144370/workflows/d5b7164c-53ba-455f-8bc5-95e9eabc7ea4/jobs/777276

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Some notes:

• The compile time did not increase, it's ~65000 ms in both
• Return times don't seem to have been changed either

Search keywords:

[DiegoAndai]

@oliviertassinari this looks related (reference):

Average CPU usage down to 25% from 75% in around the same dates. Do you know what could cause this?

[DiegoAndai]

It's definitely a React 19 thing:

In this PR #44868:

• Ran with React 19 in 20 minutes
• Ran with React 18 in 4 minutes

[DiegoAndai]

Looks like it's related to the Suspense changes: #44935

[DiegoAndai]

Hey @eps1lon, what might be going on here? After upgrading to React 19, our regressions tests went from 4 minutes to 20. If I remove the Suspense component from the test (see #44935), the test returns to 4 minutes. The Suspense component was added here: #28239.

I couldn't find much information about the impact of the Suspense React 19 changes on React.lazy, so I'm lost as to what could be the cause or possible solution. The tests also take 20 minutes with React's 19.0.0-rc.0, so this doesn't seem related to the latest changes made to Suspense before the stable release.

If you have any ideas or could point me to a resource that might be related, I would greatly appreciate it.

[eps1lon]

After React displays a fallback, React will throttle the reveal of the Suspense boundary by 300ms to reduce jank (see facebook/react#31819). Should be fixed by wrapping the initial render in React.startTransition.

[DiegoAndai]

Thanks for the response @eps1lon. I don't understand what you meant:

Should be fixed by wrapping the initial render in React.startTransition.

To which initial render are you referring to? Where/how should React.startTransition be used?