I think it has to do with how much data is on your iPhone. The transfer process must make a determination of which data it can transfer from iCloud and if that is the most efficient method. I also have some specialized apps which require downloading and storing additional large dataset downloads (400GB+). My suspicion is that this is what causes the long multi-hour migration process. In the future, I’m going to make sure to remove those apps and their data and just re-download those specialty datasets because my internet connection seems to be a lot faster than the bluetooth/wifi transfer that the migration process is using!