[deleted] I was wondering if we get a delta update of a small size, like 1MB with an instant download, how is the installing part taking so much longer.
I believe most modern Android systems have two entire system partitions, the "A slot" and the "B slot". At any given time, either A or B is marked as the one to boot. A delta update says how to transform the currently-active slot to produce a new system image. If the active slot is A, then the updater copies the entirety of the A slot partition to the B slot, then applies the changes to the B slot, then marks the B slot to be tentatively booted next time. If the tentative boot works ok (as opposed to crashing), then the B slot will mark itself as the active slot.
Having a system for tentatively booting a post-update image means that if something goes surprisingly wrong (perhaps due to a weird mix of installed apps on a particular device, who knows?) then there is still a working system that can be booted. If the delta update is observed to fail for a significant number of users, then that update can be replaced by a different set of change instructions on the update server, at which point hopefully applying the new update to the currently-working system will result in a different new system image that hopefully will pass the tentative-boot test.
If I have that right, then most updates -- whether the downloaded delta was large or small -- will take roughly the same amount of time in the "copy and apply" step.
I'm sure I'm leaving out many things, e.g., signature verification, but also integrity checking (aside from the delta process, the entire new system partition needs to be checked for integrity regardless of the size of the delta update).