I was just troubleshooting very slow download speed and discovered one possible answer, so thought I'd share for those who find this in Google as I did.
From what I can determine, Galaxy downloads and installs the files simultaneously. If the game has a ton of small files (Nexus: The Jupiter Incident is what prompted this), then disk I/O and memory cache space will limit the effective download speed, sometimes severely. However, since you would still have to install the game after downloading, which would involve the same operations, the net effect is nil. Unfortunately, since the client gives no indication of this, it is hard to tell what's really happening. If you're using windows you can take a look at disk activity and latency in Resource Monitor to confirm. On Linux iostat will provide similar info. Anther telltale I noticed was highly bursty network throughput, as the memory cache would fill and empty.
I happened upon this because at the time I was running on a cheap Windows convertible I specifically use for retro gaming on the couch, using an SD card for storage. This is about as worst case as it gets for small file throughput, but this is likely to be a factor for rotational disks as well, especially if combined with low RAM. Disabling A/V may help, as it should reduce I/O load.
Enjoy!