new iMac setup taking unusually long I just unboxed a brand new iMac and after selecting language, keyboard, iCloud stuff it's now been on the "Setting Up Your Mac" screen for an hour and a half.

It's connected to my home wifi network which is online and reasonably fast. I have an iCloud account but don't do anything with it.

Is it downloading some huge update?

Do I let it run? Turn off / on again?
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.