For what it’s worth, same issue here with a 2015 iMac. Boot time is more than 5 minutes. Even after logging into the user account it still takes several minutes to get to a responsive desktop, with the dock, menu bar, background appearing one after the other, the screen going black, mouse not responding, etc. This is like a time travel experience back to Windows 95. Once the desktop is up and the first apps are launched, the machine runs fine. Maybe a bit slower than Catalina (which can be expected with a new OS).