Shared libraries

I agree that compared to classical package managers it's impressively fast.

But no, Docker's deltas aren't even optimal for updating your application from one version to the next. They are a delta between one layer and the next one, so only efficient when the last few layers have changed.

Fast updates are often a symptom of applications not updating their base image with security patches as often as they should.

Docker could be a lot faster even, if it computed deltas between versions, or had a protocol like rsync or casync that allows figuring out the delta on the fly.