same issue here but after a week of browsing trying everything, i solved this finally!!!!!!!! so i got imac 2014, 8gb ram version core i5 full story here everything was okay, updated to big sure, lovely, then a day later everything started becoming unresponsive, type a letter, click on anything, it just frozen with the rainbow wheel, takes half an hour to startup so i reset the imac, clean re-installed big sure, good for a day, then same thing started happening. so i downgraded to mojave, good for 2 days, then suddenly samething again! anyway, days of trying different things, reset smc, pram, removing one app after another, disable login items etc etc, i came across some one saying remove Avast. Thats when it hit me, in big sur i had avast installed, then one restart later, the imac slowed down, same on mojave, so i removed Avast, now my imac is working perfectly well!!!!!!