Why do Hyper-V VMs Show a 10Gbps Network Adapter?

If the traffic is between two VMs on the same host, then the traffic never touches a physical network adapter and the traffic will process between them as fast as the VMBus and processor can handle the traffic.

If the traffic is external to the Hyper-V host, then the speed is based on the speed of the network adapter (or adapters, if a team) and also the processor. For example, if you have a 10Gbps network adapter, the speed will likely be determined by the processor, which has to process the traffic, so you might not actually see 10Gbps of speed. When receiving traffic, each VM NIC might be assigned a VMq from the NIC, which is processed by a single processor core (except in Windows Server 2012 R2, which supports virtual Receive Side Scaling, vRSS). This likely will result in speeds between 3-4Gbps.