Take a look at your systems resources. Do you have at least 1/3 of the HD free? Have you tried defragmenting your HD? If your system is highly fragmented it can cause slow file transfers. You can also have a drive thats starting to fail as well.
Try connecting via direct Ethernet to your router, Is that any better?
What are your iPhones? You maybe able to download directly over the cellular network (check your service contract first! Some limit you on how much as well as how fast).
Don't forget your internet connection is not symmetrical (same data rate up & down) must are asymmetrical (very slow up and faster down). Why this is important is if you are saving to your iCloud account. Make sure you don't enable iTunes Match services until you've upgraded your iPhones.
Check your internet connection using this free app: Network Speed Test.