Bug: render time differ depending on load

[freeboub]

React version: 18.2.0

Steps To Reproduce

1/ Create a heavy component to display

can be simulated with something like:

const HeavyComponent = () => {
const start = performance.now()
let i = 0;
while (i < 9000000) {
i = i + 1;
}
console.log('render time', performance.now() - start)
return null;
};
`
2/ force refreshing this component periodically with a setInterval.

const App = () => {
// eslint-disable-next-line no-unused-vars
const [_time, setTime] = useState(0);
// setState in an interval to for refresh periodically
useEffect(() => {
const interval = setInterval(() => {
setTime(_time => _time + 1);
}, timeout);
return () => clearInterval(interval)
}, []);
return <HeavyComponent />;
};

BUG
Depending on the timeout configured (ie refresh frequency) the execution time of HeavyComponent differs (one to the double).

Output in console:
When putting a low timeout (10ms), the average render time is around 2.7 ms
When putting a high timeout (100ms), the average render time is around 6 ms

A very basic sample sample is available here (with the code I put on the ticket):
Link to code example: https://github.com/freeboub/ReactSampleForDevTool

The current behavior

The execution time is not consistent depending on app load

The expected behavior

The execution time should be consistent depending on app load

additional informations

• I was investigating this issue in a react native project. -> issue highlighted with react dev tool.
• The result are consistent with profiler output (I can share screenshots if needed).
• Changing the HeavyComponent render time also gives consistent one to double refresh time.
• I was investigating this issue with profiler, but as console.log also highlight the issue, I kept it.
• There is maybe an explanation for this behavior, but was not able to find it in bugtracker nor on any other internet link.
• Same behavior observed with
  "react": "^19.0.0-canary-33a32441e9-20240418",
  "react-dom": "^19.0.0-canary-33a32441e9-20240418",

Thank you !