I've got two USB-based network adapters laying around here: the official Apple USB 2 Ethernet Adapter and this USB 3.0 ethernet adapter based on the Asix AX88179 chipset.
Then I connect over a VPN, and use scp to see how much time it takes to pull over a 100 MB test file. With the Apple adapter:
$ scp testbox:~/tmp/a.log . a.log 100% 100MB 3.0MB/s 00:33
And now with the USB 3 based adapter:
$ scp testbox:~/tmp/a.log . a.log 100% 100MB 2.9MB/s 00:34
Huh weird, no difference. I'll go and check where the bottleneck lays. VPN? scp?
Not using VPN gives the following result:
$ scp gateway.company.tld:a.log . a.log 100% 100MB 16.7MB/s 00:06
That's somewhat better. Testing without scp, but with wget instead shows roughly the same number:
$ wget http://www.company.tld/some_video.mov 100%[====================>] 126,805,954 16.0MB/s in 7.6s
Now I test it on a different machine, within the company network:
$ wget http://www.company.tld/some_video.mov 100%[====================>] 126,805,954 64.1M/s in 1.9s
That's better. So the bottleneck is apparently the physical (ethernet) connection I've got, which is VLAN'ed to provide a guest network. Oh well.
Long story short: better check this guy's test results. You should be able to get up to 110 MB/s.