unstable (bi-modal) render durations

[mdjastrzebski]

Describe the bug
I have observed that in the recent versions of React/React Native render durations seem to cluster around 2 values. The expected behavior would be to cluster around single value (i.e. the expected render duration).

Example results:

Mean: 78.2 ms
Stdev: 23.3 ms (29.8%)
Runs: 83.8 52.4 52.6 79.9 106.7 105.2 56.4 53.9 67.3 105.6 105.6 70.5 53.6 54.5 107.5 106.6 85.1 53.8 57.3 105.5
Warmup runs: 105.0
Removed outliers: (none)
Source: #584 (comment)

There is one cluster around 52ms and secodn around 105, with some results happening in-between.

To Reproduce
Run complex enough test (e.g. AsyncComponent from Reassures test-apps/native) under recent:

• React Test Renderer 19.1+
• React DOM 19.1
• Test Renderer 1.1 (which correponds to React 19.1)

The results should cluster around 2 separate values.

When running under React Test Renderer/React DOM 18.x or 19.0 or Test Renderer 1.0 the results cluster around single value.

[mdjastrzebski]

So far I've isolated the issue to be releated to owner stack features in dev builds of React (dev builds are used in tests).

• 32b0cad8f7 enabled the feature (previously behind experimental flag) - however it affected all runs uniformly (= slowed down)
• 4a9df08157 introduced limit of 1000 stacks to be created in given inteval, which directly leads to bi-modal results.

[mdjastrzebski]

If you are using Reassure with RNTL v14, a temporary workaround is to use test-renderer package in version 1.0, which uses React Reconciler with version before the mentioned above changes. There is currently no workaround for RTL or RNTL v13.