I know how it works on Steam and i think it is not different on Galaxy.
If you download a game its compressed and the higher the download rate the more data needs to be extracted. That is causing your high CPU usage and makes the CPU to a bottleneck at high download rates.
True, but it have also happened when just downloading the installer. Nothing is extracted, it's simply just downloading the files.
The client also used to be hogging a few percent of the cpu when doing nothing.
Not particularly versed in the topic downloading..... But I guess downloading in a high speed is more resource expensive than what one may think. Went from 100/100 to 250/250 earlier this year and I'm seeing higher cpu usage during downloads than what I used to see.