IIRC, iOS updates are effectively delivered as an entire filesystem image. So when the update is being performed, the old root filesystem image is being replaced with the new one. The exception is when it’s an OTA (over-the-air) update, where only ‘necessary components’ are downloaded. See https://help.apple.com/pdf/security/en_US/apple-platform-sec... page 59.
I don’t know if ‘necessary components’ means a binary diff or something bigger. But if a security update requires rebuilding multiple components, that diff could still be large.
So, you’ve got either an entire image being written, or an image being patched. Then it’s all checksummed and verified. Then you have a phone reboot, with any necessary ‘run-on-update’ scripts being executed.
I don’t know if ‘necessary components’ means a binary diff or something bigger. But if a security update requires rebuilding multiple components, that diff could still be large.
So, you’ve got either an entire image being written, or an image being patched. Then it’s all checksummed and verified. Then you have a phone reboot, with any necessary ‘run-on-update’ scripts being executed.
I think that’s why it takes so long.