Megabyte or Megabit?
If its Megabit (Mb) you have a problem.
If its Megabyte (MB) its completely normal. DSM uses the same connection/cable twice, first from the VM to the router and then from the router back to the host (your PC). To fix this you have to use a virtual switch but i have no idea how to create one in Hyper-V.
If its Megabit (Mb) you can try to disable "Virtual Machine Queue", if that still doesn't fix it make sure your router, cable and your host (Your PC) Ethernet adapter support Gigabit Ethernet.
Source: viewtopic.php?f=2&t=3191#p18792