I restored my new iPhone 13 Pro from my old one (phone to phone transfer) fine, and now the photos app is slowish, as you can see, it's constantly recreating thumbnails when scrolling and also after opening a photo. My old 8 is faster, it's also faster presenting the high resolution of a photo after opening it. Maybe I need to let it attached to the power more? Or I can try another kind of restore... I don't use iCloud photos. I made a short video: