Fast upgrades

Fast upgrades is a specification that explains how could ubuntu make upgrades faster.
For reducing updates time on ubuntu, we need to lower the time needed for the longer steps in the upgrade.
From my experience, the longer step in the upgrade is retrieving needed packages. Those packages, often don't have a lot difference with the previous version of it.
A great way to improve upgrade time would be to download only binary diffs of the packages incrementally. I have seen a long time ago SuSE have something they call patch.rpm, these are binary patch for rpms installed on a system.

A second step would be that during the upgrade we gather necessary task to do after the upgrade and launch them once at the end: For example, when installing a font, we need to regenerate the font cache. Often during an upgrade, you install several fonts, then, the font cache is regenerated several times during the upgrade process. This could be done once at the end of the process upgrade. Same applies for kernel modules that runs mkinitrd, or kernels that runs grub-install.