The iCloud backup built into the iPhone for me seems quite seamless. As I mentioned in another comment, I have several thousand photos and videos that don’t fit on my phone because it’s only a 64 GB device. It’s been this way for many years. But I can still scroll back and see all the photos. And when I click one I view it instantly. I’ve never noticed any lag or any problems associated with the cloud backup. It feels to me exactly the same as if they were local.