Imac (Retina, 27-inch, Late 2014) 4 GHz quad core Intel Core i7 32 GB 1600 MHz DDR3 Amd Radeon R9 4GB The same problem here with Big Sur. It was ok for 3 month. Then 3 days ago, without reasons, it started taking too long to start up the computer, and every time i want to do something simple like a new folder, it takes between 5 min. to do it. Black screen on start up, and after put the password i can see my desk only after almost 10 minutes. It's very painful. I've tried everything but nothing helped. It seems like some process doesn't working. Apple do something now!!