Someone wrote before me: remove the GOOGLE ACCOUNTS from System Preferances >>> Internet Accounts… and… it works for me! To be shure i reset the PRAM and SMC (did this only to be shure, like i said before). I am waiting for you to write if you had the same result as me (elimination of the lag). Thanks!

Same here! MacBook Pro late 2015 The mighty Fucing i7 !!!!!! 16 GB RAM 500 SSD Dedicated video card. And... so what!?!? This is bu.......lllllll....s.........hoooootttttt!!! This is unimaginable. We are worst than Windows!!!!! I reinstalled BigSur again from zero! Erased the HD and installed. Restarted the SMC and the same s....h.....ioootototo! When i browse Youtube is EXTREMLY LAGGY and JITTERY! INUTILIZABLE!!!!!! WTF! When i preview a JPEG the animation of opening the image is jittery. THIS IS THE SAME WHAT THEY DO WITH THE iOS till thei was caught cheating! THEY SLOW DOWN THE MACHINE TO BUY ANOTHER ONE NEW! P.S. Sorry 4 my english!