Hi!
There could be performance issues once you have any webpack processing for our umd code.
It may use some polyfilling/transpiling for internal ES6 async methods which would be a reason for result performance degradation.
What we saw previously is that using IE11 umd package config for webpack surely affected on performance, so the solution above was to rebuild umd without any polyfills (like regenerator-runtime) in there.
You may compare the umd bundle performance between Scheduler 3rd and 4th version in pure JS app.
Some tips could be found also here to use patched react bundle for this.
https://www.bryntum.com/docs/scheduler-pro/#SchedulerPro/guides/integration/react.md#cra-performance
It was yet related to SchedulerPro but seems it could be also the reason for the problems in React app for Scheduler.
Looking for a runnable test case app from you to get the performance results.