Time to update this old dusty thing! I figured this is worthy of a blog entry, as it's not exactly something you can google quickly at the time of this writing.
Let's talk about network speeds with Hyper-V. I have been testing Server 2008 R2 with Hyper-V enabled running a virtual Win7 'guest' and for the life of me couldn't figure out why a member server hosting file shares was getting a ping response time of 30 to over 200+ms. Doing my research pulled up a dozen or so similar instances of network slowness and everyone seemed to attribute that to the TCP chimney offloading and disabling this in the operating system as well as on the physical and virtual network adapters will correct the problem instantly.
Well, it didn't. Turns out after playing with several of the settings within the Broadcom network adapter and recreating the virtual network for the VM's half a dozen times that the real culprit is a feature called 'Virtual Machine Queues'. If any of you out there are running into any issues with your VMs' network functionality, you might try disabling this (along with the TCP offloading, or chimney offloading) and see how that works in your case. It made an immediate difference in mine and now my response time is <1ms as usual and as is seen in any physical box on the network.
I'll flesh this out a bit later if I have time, I just wanted to get this down here before I forget and hopefully I'll have helped a few others out. Turns out I may need to dig further as that feature should actually increase performance according to this whitepaper: http://www.dell.com/downloads/global/power/ps1q10-20100101-Chaudhary.pdf