The solution to the problem turns out to be... The Broadcom NICs in this Dell server have a setting "Virtual Machine Queues" that defaults to enabled. Once disabled, the guest OS has the same network speed as the physical machine does.
Dell PowerEdge T420, Windows Server 2008 R2 with Hyper-V installed and a single 2008 R2 virtual machine setup within the Hyper-V environment. The two NICs on the T420 are connected to the same Gigabit switch, one for the physical machine's use and the other for the virtual network. The only thing being done in the VM is Remote Desktop Services and then end-users running a medical records program which is accessing a database on completely separate server. The physical server just has Hyper-V, a domain controller and DNS. With no activity, ping'ing the guest from the physical machine or vice versa, or a separate database server from the guest machine, we get under 1 ms response times. However, if there is any activity in the guest OS, ping times vary drastically -- between 1 ms and 200 ms.  I have the various Offloads disabled in the virtual network adapters properties but that does not seem to make any difference. Looking at the Resource Monitor on the virtual machines, there is generally under 5% of the CPU and 10% of the memory being used, and network traffic when the application is running is under 100 Kbps (usually under 50 Kbps). On the physical server, the CPU & network traffic are basically the same as the virtual machine, with 80% of the memory in use (since it is all allocated to Hyper-V). Any thoughts on how to solve the slow networking? Thanks in advance!
Hyper-V networking slow