Like everyone else, I am having the same issue. However, as some others have said, the issue does not seem to be entirely with restoring from iCloud backup. When I initially did set-up, I used the phone-to-phone method. I let it run, and run, and run, and...
After over two hours (with "19 hours remaining"* and little or no progress on the progress bar), I spoke with Apple via chat. The agent had me cancel (which, of course, wipes the phone) and restart but using the iCloud backup method instead. And now have this issue where the screen becomes non-responsive and then going to the black screen with the spinning status icon (looking like the phone is doing a soft restart)... it will then become (briefly) responsive.
I'm on my 3rd or 4th try and it currently says 200Mb to go... so seem to be making more progress this time. But time will tell.
*Obviously, internal network and, for the download from iCloud, internet speed can impact. I have a fibre internet connection and a speed test of my home wi-fi from another device shows download speeds of approx 450mbps and uploads a bit lower... around 300mbps from memory. At any rate, my understanding is the phone to phone option takes place locally so internet performance shouldn't be a factor for the phone to phone transfer... in theory.
** Both old and new phones are 512 GB storage (and the old one was about 50% full so 256 GB of data). Two hours seems reasonable for the entire transfer. Two hours and little or no progress does not. :-D
