I like many people here had the issue of slow downloads due to bad DNS settings. In my case it was much more frustrating, as I'd learned about openDNS a while ago and had installed a proxy caching server (which defaulted to openDNS, in their docs and installation I was made aware of this). So here I thought, shoot! Simple fix, change the DNS in my network settings so connect to the best one around. So I disabled my proxy, re-enabled my desktop for automatic search, but to no avail. Thinking rats maybe there's some other issue with my computer, I kept digging and digging.
However I got curious, what DNS exactly did my computer pick when I left it run wild as opposed to the proxy? My internet speeds were virtually untouched switching from my openDNS proxy to window's automatic selection and that just seemed weird. Suprise suprise after cracking open cmd and using ipconfig /all I found that instead of hooking up to openDNS, my computer determined that 8.8.4.4 was the best DNS. In short the settings jumped from one bad example to use with iTunes (openDNS) to another bad example (google's DNS).
If you have any reason to suspect that something is in fact up with your DNS, because for whatever reason your internet connections are fast, but iTunes is ungodly slow here what I found to be the best tool to hook up your computer to a DNS likely suited for you. https://code.google.com/p/namebench/
Be aware the testing process will take a while, just leave it be, when it's done it will open up your internet browser with some stats, the important part though are the two ip addresses in the top corner, they described the closest and fastest DNS servers near you, use those in your ip v4 settings, done 🙂 see if you don't get results.
Have a good day all.
For reference my download speeds before I figured this out: ~0.2 mbs, after? Actually I'm not sure I wasn't paying attention after I got results but it managed to download a 1.52gb video in about 12 minutes, you're free to do the math, I'm just happy.