After quite a bit of research and having this same issue, I have what appears to be the resolution (at least for me) it appears to be Microsoft Security Essentials treating the "green" loading screen as a "questionable download" and doing a scan on every byte of info coming in. You might also notice your hard drive filling up as well. (Mine used about 40g just in loading the game). Since I have AVAST I uninstalled MSSE and now the "green loading screen has reduced for 20 min load time to about 2 min.

Hope this helps...