I have iMac (Retina 5K, 27-inch, 2019) and since installing Big Sur my boot times have increased by at least 300%. For the first 5 minutes after boot my iMac is unresponsive and any attempt to start an app takes around 5 to 10 minutes. I rebuilt the fusion drive and performed a clean install of Big Sur and this made no difference. My iMac is less than a year old and is almost unusable. My Activity Monitor shows my system at 90% idle so I have no idea what is causing the poor performance.