Bug ID 6486234: The virtual networking infrastructure adds additional overhead to communications from a logical domain. All packets are sent through a virtual network device, which, in turn, passes the packets to the virtual switch. The virtual switch then sends the packets out through the physical device. The lower performance is seen due to the inherent overheads of the stack.
Workaround: For T2 platforms, you can assign a Network Interface Unit (NIU) to the logical domain.