There are multiple factors that go into this. Main one on release day being load balancing on the servers. It also doesn’t help that major releases are typica

There are multiple factors that go into this. Main one on release day being load balancing on the servers. It also doesn’t help that major releases are typically done by downloading the full image instead of your typical delta update. Add onto that the process where it creates a compressed backup of your current version for rollback purposes and the verification process to check that your download didn’t end up corrupted and it’ll keep your iPhone busy for a while.