I got rid of this warning by changing "comp-lzo" to "compress lzo" in my config file as noted in the link below: https://github.com/Nyr/openvpn-install/issues/430
USB doesn't seem to be an option; why not add a gigabit PCI card for ~$20 or so?
http://lime-technology.com/forum/index.php?topic=35884.msg334412#msg334412
http://lime-technology.com/forum/index.php?topic=12903.0