Comment 5 for bug 1825812

Regarding the optimal behaviour u-u spends significant amount of CPU time on deciding which are the upgradable packages especially if there are held packages thus there is a tradeoff between network and CPU usage.
I believe both of the options have their benefits, and on a desktop system -updates are more likely to be installed quickly after downloading them thus software-properties seems to be doing the right thing by requesting all packages to be downloaded.