You'll find some recent posts above that address this, but in short: the server is very busy at peak times, resulting in slow downloads. Lots (many hundreds) of non-Premium users are sharing most of the bandwidth, with a portion available for Premium downloads. So while we're not throttling non-Premium users, the much smaller number of Premium users doing downloads do get higher speeds.
We are hoping to cure the issue but there is no timeline for this, yet.
Meanwhile, non-Premium users can generally get high speeds by downloading at off-peak times.