Catalina 10.15.7 (19H2)

There are many system processes all starting up at the same time and competing for CPU time when you first power on the iMac. Within a few minutes, most of these processes will complete, and your CPU usage will drop to single digit usage in Activity Monitor — if you are not running other applications.

In my case, it was a strange error within iCloud sync. I stopped network, so I can temporarily use cpu, and I checked out almost every iCloud syncs. Then back to wifi, and the problem was solved.