Anyone got any ideas what would cause particularly slow network (samba) transfer speeds with a Hyper-V VM?

System is:

Main server running Hyper-V : Windows Server 2008 R2
VM running on server : Ubuntu server 14.04

Copying a 1.4 GB file from an SMB share on the Win2008R2 server to /dev/null (so not actually writing) I get a speed on the VM of : 13.9 MB/s

On a separate system on the LAN (all gigabit) I get a speed of : 49.8 MB/s

I did a couple of other tests too. So all tests:

SMB-Main Server to VM : 13.9 MB/s
SMB-Main Server to other system : 49.8 MB/s
SCP-VM to other system : ~16 MB/s
SCP-VM to other system with weaker cipher : ~20 MB/s