Seagate USB '3.0' Speeds with Ubuntu ZFS

Question

I am currently researching zfs. I have a RAIDZ-2 system with an SSD Cache. My issue is backups, our idea is to send snapshots to a USB 3.0 Seagate external harddrive.

I understand and have read the many issues of USB 3.0. It is a USB 3.0 device and I am using a USB 3.0 PCI Express card powered by Molex.

When I send a snapshot to the system, its very slow. I use this command below:

zfs send test/home@snappy | pv > /mnt/usb/hello

Pipe Viewer gives me an output of 37.5MB/s average transfer speed. hdparm also gives me a read speed of 22.10MB/sec.

Now, many and I may blame this on the device itself or the RAIDZ it is getting the snapshot from. However, The command below:

zfs send test/home@snappy | pv > /dev/null

Gives me an output of 335MB/s. So it cant be RAID slowing me down, right? Also, when i plug the device into a Windows machine I can get speeds of around 80MB/s. Another note, I have tried these tests on two Seagates, one ntfs and one ext4 format.

My question is, what could be slowing the device down so much?

Thanks for your patience and any suggestions. Please ask for any other information.