I had the same issue and finally managed to fix it, at least for me. I have 2019 16" almost maxed out configuration MacBook Pro. Everything was super smooth until I upgraded Mac OS to BigSur. After upgrade all animations were lagging. Especially moving the windows around was very laggy and the issue was especially bad when I had external monitor connected. I tried multiple things including SMC reset, NVRAM/PRAM reset etc. These helped a bit sometimes but the issue came back within minutes. Finally what really worked was to completely factory reset my MacBook. I went to recovery mode and first erased all volumes and then reinstalled Big Sur. After fresh install the lag went away and everything is super smooth again, even with external monitors connected. Can't say it will work for all, but for me it did the trick