new iMac setup taking unusually long
That's odd. Since you've put nothing on it, you should just hold the power key and start it up again.

If it hangs, you can boot to recovery and repair the hard drive catalog structure. Nothing should download until you log in to the first account and set automatic updates, etc..

You can isolate it from all network and iCloud settings and just set up a test account. It's easy to clean a user out and start over with the "out of box" assistant once you're in to the first user account without any network setup.