Manual replication via USB3 drive very slow

When running a manual replication to an external USB3 drive, I am seeing data transfer rates of only around 150Mbps over our gigabit LAN. If I start a second transfer, the total speed will roughly double with each transfer still running at 150Mbps.

Does this transfer speed sound about right? I have no problem saturating my network connection when copying files from a windows network share hosted on the same array.

Is there anything I can do to speed thing up a bit?

Thanks