We'll start looking at this in the next day or two then we'll know more. If it's something low-hanging, perhaps this can be part of v5.4.
Does it get better if you remove these configs?
autoRowHeight: true, minRowHeight: 100,
@animal if you try using the calendar with 2000 events, it will go slower. On shift next & previous it depends if in the next month there is less data then it quick loads up. The code that i have shared with you is only creating the events for month of june. may be you can create around 2000 events per month starting from april to august and then shifting among these months will reproduce the performance issues. The calendar version that we are using is 5.3.1. But the performance issues I am recording at https://bryntum.com/products/calendar/examples/megadataset/
I am unsure of the version used in your above example page.
Here is the demo video: