Comment by daneel_w

It's been doing this since long before iOS 18. I first observed this behavior already on my 1st gen. iPhone SE with iOS 11 or maybe iOS 12. Every now and then, especially after having taken new pictures with the camera, I'd see the network activity spinner get going whenever starting Photos - and I've never enabled iCloud backups, but rather explicitly always disabled it everywhere I find a setting to do so. With Wi-Fi it'd be a short one second affair, and on cellular it'd take a few seconds more. I also observed the duration of the activity to vaguely extend with the number of new pictures taken between starting Photos. Curiously, when disabling cellular data permission for Photos and having no Wi-Fi connected, the network activity spinner would still pop up and just get stuck indefinitely without ever finishing whatever it's reporting.

My gut feeling has always been that it's returning digested machine learning feedback for Photos' facial/object recognition.

(add.: since moving from the iPhone 8 and iOS 16 to a modern iPhone and iOS 17/18 this behavior has gone away - at least the network activity spinner doesn't gossip anymore...)