Let's say there is to be done:
54 packages upgraded, 1 newly installed, 1 to remove and 1 not upgraded etc.

Idea: well in fact, these can be smartly grouped:

First split the list into groups of packages that don't depend on each
other.

Then download the smallest (bytes) group.

Then start installing them.

But even when we are still installing them, start downloading the next
group. Etc.

OK. It is too confusing to do parallel installs. But at least
downloading of independent (of this group) packages could continue,
while installing is occurring.

Well, perhaps all this is only important in slower speed countries.