There’s a good chance part of this is face recognition/etc on your photo library, which is done on device, not in the cloud, and not synced to the cloud either to protect the privacy of the people you take photos of.

If you have terabytes of photos to process, it’s going to take a week, and it’s going to get hot, and it uses too much power to run on battery power, and of course the battery also gets hot while it’s being charged.