Transferring 2.4 TB of data; bottleneck on backup was about 400 Mbps (standard SATA HDD speeds). On restore I was only getting 200 Mbps. I thought perhaps it was the ZFS (that somehow read operations were twice as fast as write) and stopped investigating. As I was lying down with my head on the table (swollen jaw – wisdom teeth), I noticed I’d somehow unplugged the USB cable from the USB 3.0 socket and put it into the USB 2.0 socket.
Surprise, surprise, this was the bottleneck.