An additional parameter to have a play around with in the Advanced properties of the Atheros NIC would be to disable 'Flow Control' and see if that makes a difference.
I'm assuming this is the first time OP has had a 100/20 UFB connection. Are you in a main centre or smaller town? I understand if an ISP has 1 Gb/s instead of 10 Gb/s handover it can make it harder for speed test to look good. You might need to test by running a well resourced P2P torrent or similar. Who is Stuff Fibre's wholesaler these days?