Taking forever to sync iCloud with new Mac

Syncing with iCloud doesn't usually involve anything spinning. Synchronizing is a background operation. Look at the "Force Quit" Menu

and see if there is an app that is "Not Responding." If you find one, force it to quit.

(If you're inside a "Not Responding" app, then the Apple menu may not work. Change to another app by clicking outside the app or by using the "Command-Tab app switcher.") Or use command-option-escape ( ⌥⌘escape .)

Are you really using macOS 15.0? You should upgrade to 15.5-- there's lots of improvements!

Anniego wrote: How long should it take?

That would depend on how much you're syncing, how much memory and storage you have, what your settings, especially concerning Optimizing. Fully stable after maybe a week or two if there's lots of stuff.