Most likely, you are seeing an issue avec Virtual Machine Queues.
If you are running a Broadcom network adapter, vous devrez disable Virtual Machine Queues. We juste setup a nouveau server, running Server 2012 Standard avec Hyper-V, running two Server 2012 Standard guests, et saw le exact même thing you are describing. This seems to be an issue affecting Broadcom adapters.
Go into le Hyper-V host network properties, et look at le physical adapter, pas le vEthernet. On le Advanced tab, scroll down to Virtual Machine Queues, et set it to disabled.
Once we made this change, things were flying.