After upgrading my MacBook Pro 13" (2017) to Big Sur in Dec 2020 I had a similar experience. I use a 4k monitor and everything got super slow, fans started spinning, processes got throttled, and I could barely get any work done. I later did a fresh install of Catalina and everything was fast again.

I never did proper benchmarking, but my feeling was that resolution played a role. When using 1080p things were fast, but the resolution is unusable on a 27" 4k display. When using 2160p things were fast, but too small for my eyes. Any resolution in between (this implies things are being scaled?) was sluggish.

Note that this is a 13" model, so the problem does not seem to be restricted to the 16" model that the author is talking about.