Have tried Cubase 11 on an Intel Mac running Big Sur 11.1 and it runs fine there, so my theory is that it must be related to Big Sur 11.1 on Apple Silicon/M1. Cubase 10.5 and 10.0 does not have the same issue and seems to be running fine.
I might try 11.2 beta on my M1 MacBook Air over the holidays to see if that changes anything.
It is early days for Apple Silicon and I know that Cubase is not officially supported on it yet, but from what I have seen performance wise so far with Rosetta2, I am really looking forward to the power of a native M1 Cubase version.