Sorry that this is a bit late but I've had a couple of instances where it seemed that the network performance was sluggish under Hyper-V 2012R2.
We have found the problem to be with the Broadcom network cards and their handling of VMQ. I believe that there is a firmware and driver update to fix this but have found that just disabling the "Virtual Machine Queues" under the properties of the network car on the host fixes the issue.