* Update * - Here I use the term TCP Offload. The actual feature is properly termed TCP Task Offload. TCP Offload is better referred to as TCP Chimney and that should never be turned off.
Here are some tips that I have passed on regarding Hyper-V and TCPOffloading.
In theory TCPOffloading should always make things better. It takes work away from your server OS, thus everything should work better. Right?
As a former System Administrator, I have been disabling TCPOffloading on Windows Servers for years. Always to fix network performance issues. This works for SQL servers, Terminal Servers, and now Hyper-V servers.
Why? I honestly have not gotten deep enough into the issue to fully understand why, I just know that if your network throughput is pathetic on a Windows Server that is running some application or role, try this trick.
(as always take caution, this might work, it might have no effect - I take no risk from you)
In this case, focus on the NIC driver on the base network connection. (not the virtual one that your parent partition uses, but the Physical NIC properties (the NIC with only the Microsoft Virtual Network Swith Protocol enabled))
Make sure it is either the WS08 provided driver, or the latest from the manufacturer (it is usually always safer (read this as: overall more stable) to use the one that ships with Windows - unless you need a feature provided by a different driver)
Try turning off TCP offloading - as this can cause strange behavior on some networks.
Also, with Hyper-V if you are using a manufacturer provided driver and you want to do fancy teaming and stuff like that at the driver level - please make sure the driver has been updated for Hyper-V, or else you will get weird results here as well.
Don't use NIC teaming, or NIC management software unless the manufacturer says it is Hyper-V role happy (not just WS08 happy).
Some NIC teaming management software needs special control of the hardware and it might not work after Hyper-V is installed, since the physical hardware is now shared.