Just a heads up, not sure if it this is just isolated to my Lab or if this could be a nice little issue with Hyper-V 2012. Over the weekend I was playing with a few different firewall solutions trying to find a replacement for my limited Sophos UTM firewall (50 IP's max), So I built a VM which I had to change the Network card to Legacy instead of the Default, after which I started to have nothing but random slowness, Internet would stop working, or be extremely flakey. Last Night I deleted the "Powered OFF" VM and everything started working perfectly again.
Sorry for the short post, on the run and did it from the good old iPhone :)
Thanks for reading