The problems I have had since many weeks (!) in BigSur on my beefy MacBook Pro 15 was: Extremely lagging UI just everywhere, in Safari, in Finder, Word, Chrome, Mail etc. It sometimes took 10 seconds until the UI responded to a click or a key stroke. For example when writing in Mail it took many seconds until the letters show up. Also in video conferences there were stutters, also reported by other people who joined the sessions (I have Fiber 1Gbit/s connection). I tried: detaching dock detaching external screen restarting the system (sometimes it solved the problem for some hours) closed all safari tabs (it was helping a bit) moved thunderbolt cables to other side of laptop checked with Intel Power Gadget the performance checked activity log, top etc (no evidence) installed temp monitoring, nothing special cleaned all caches with OnyX reset SMC reset NVRAM or PRAM There was zero clue what what going on when the system was so unresponsive. What solved the problem immediately, close my 10+ Finder windows!! And then the system went back to normal. (Maybe there is a problem with the preview option in Finder??) For your reference, I have installed BigSur 11.2 (20D64) and using a 2018 MacBook Pro: MacBook Pro (15-inch, 2018) 2.6 GHz 6-Core Intel Core i7 32 GB 2400 MHz DDR4 Radeon Pro 560X 4 GB Attached is a CalDigit UBC-C Pro Dock (Logitech Mouse via USB transmitter, USB headset etc) and via another Thunderbolt 3 port my Acer Nitro 4K Display. I hope this helps.