Recently, I’ve been working on several issues where customers reported poor network throughput. This happened on recent and performant hardware, that showed in benchmarking the 10GBit Ethernet connection, or slow Live Migration. The throughput was only about 10 Percent of what is expected. As it turned out, the overall performance suffered, but network throughput was the most obvious one.
Some of this problems are addressed with this very recent hotfix:
Performance decreases in Windows Server 2008 R2 when the Hyper-V role is installed on a computer that uses Intel Westmere or Sandy Bridge processors
Other occurances could be solved with:
Hyper-V: Performance decrease in VMs on Intel Xeon 5500 (Nehalem) systems
Here you could either disable Enhanced C States in the Bios, or use the explained Registry Key to disable the C2 states.
Please note, that this will likely result in higher power consumption.
Lastly, I’d like to point you to a fix you may need after Service Pack 1, to address network issues inside a VM:
The network connection of a running Hyper-V virtual machine may be lost under heavy outgoing network traffic on a computer that is running Windows Server 2008 R2 SP1