I'm having the very same issue with my 2015 imac. I was low on RAM and increased it to 32gb and my CPU is operating fine. But on start up I'm finding first that it takes a few minutes and when trying to open apps I not only get the spinning wheel I also have messages that I do not have permission to open that particular app or that the app isn't responding. Sometimes it crashes altogether. This has all happened since the Big Sur update.