Bug: Webpack process and setImmediate polyfill interferes with time slicing

Here's the copy paste of the polyfill: https://gist.github.com/gaearon/5d2f7bedf76d7bbec2b5c1ee04861ce3 This reproes with old CRA (e.g. `react-scripts@0.7.0`). I saw this on this branch: https://github.com/Swizec/react-fractals/pull/16 The result is concurrent renders get grouped into a huge blocking task: <img width="815" alt="Screenshot 2021-06-15 at 18 01 27" src="https://user-images.githubusercontent.com/810438/122094843-8581cf00-ce04-11eb-9a40-fd3758301255.png"> Not sure why yet.