DNS Servers have NOTHING to do with your download speeds.
DNS = Domain Name System.
They only tell your Computer which IP is behind itunesdownload.apple.com (or whatever they are named, i made that one up)
Once it has this information, it doesn't care about the DNS for quite some time, as he already knows where to look for that site
The DNS is nothing more than a phonebook. As long as your computer remembers the number, he doesn't need to use it again.
Also opening up Ports in your Router OR Windows Firewall wont do anything at all.
iTunes is an outgoing connection, so they both don't block it. If they did, it wouldn't work at all.
Don't just randomly start opening up ports if you don't know what they do. It is not needed, and it wont help you at all.
The problem here is on the side of apple. And they don't care about fixing it.
I'm also running on 100Mbit, and im waiting since 30 minutes for 30MB Download of an App to finish.
Seriously, thats slower than ISDN speed was....
Only about an hour ago, i loaded the big Debian DVD Set with almost 12MByte/s. Every service runs fast for me, except anything i download from itunes. Doesn't matter if its my Windows8 Machine, my iPod, or whatever. Same problem everywhere. And its only with downloads from iTunes.