From release notes:
Following an update to iOS & iPadOS 13, users with large photo libraries might need to wait for an extended period while their device displays the Apple logo with a progress bar. (51147659)
I've seen this on a test iPhone SE device, as well as on the Apple TV. Both would stay with the progress bar untouched, but connecting to the console would show PhotoAnalysysd and related processes running.
As both devices are test devices (iPhone SE I've even restored as a new device, just in case, and the Apple TV, I did a reset (home+desktop button for 5 seconds) and let it boot and care about the photos later.
However I'm gonna wait until this is solved, as my main library has 100K photos and almost 2TB of icloud occupied, so I know already that anything that touches photos will be at least a whole week burning the device, so I'm clearly not gonna be waiting a whole week for the progress bar to finish.