It is meaningful in ways that are not measured in development time. There is a genuine effort to be less offensive regardless of how big of an impact it currently makes, or how meaningless individuals may believe it to be.
Yes, I also experienced this with my 2020 MBA after updating to Big Sur (via a clean install). Scrolling with the trackpad in Safari was especially terrible at first with jitter and the appearance of dropped frames. This seems to be better the last couple of days for me. It's still not as smooth as it was with Catalina.
I put up with it because I’ve used Mail for many years without issue until now. Fortunately for me it happens infrequently enough that it’s mildly irritating to me so far. Much less irritating than when I open my MacBook and receive dozens of iMessages at once along with their dings.
I experience this as well after upgrading to Catalina. I thought it was my fault at first, caused by some sort of key combo. It only happens for me when in full screen with another app.