I learned something today about Lync virtualization. We recommend an 8 CPU core (Enterprise class CPU) hardware profile for a physical Lync Server. However, Hyper-V supports a maximum of 4 virtualized CPU core. Due to this resource constraint, a virtualized Lync Server role can handle approximately 50% of the load compared to a physical server running Lync Server and using the recommended hardware specifications.
In addition, a 10% virtualization overhead is deducted.To find out more check out the whitepaper here.