55 minutes for the demo corresponds to roughly 1550 KB/s---not maxing out your connection speed overall, but not wasting it either.
Brief drops in speed (for small files, as you noticed) are expected---there's some amount of overhead for each file downloaded; for large files, that overhead is comparatively minuscule, but for small files, it can be significant.
There's one important difference between the demo and the updates, which is (as you guessed) that the demo packs many of the smaller files together, offsetting a lot of the per-file overhead. It generally isn't possible to do the same for updates, because we can't be sure in advance which files a person will need.