Having the exact same problem with this device. I only get 20mbps on my 100mbps fibre line when all other devices are getting 100.

Very frustrated.

I managed to get the link speed up to 433Mbps on the archer by changing the routers 5Ghz channel. This became very unstable and the link speed would jump around from 72.2 Mbps to 433Mbps.This channel change also dropped the link speed on my iMac from 270Mbps to 135MbpsSo I put it back as was.

I have an Archer C7 with a 500 /200 Mbit broadband connection.
On the gigabit wired connection I can easly get 520/220 Mbitspeeds even when other devices are using the net, mostly on 2.4GHz
With the Archer T2U, which is the only device using the 5GHz channel, upload speed can often get close to the ISP specified 200Mbit, but the download speed never goes above 150Mbit, which is roughly 30% of 433Mbit. If it would be a wifi / USB limitation, upload speed should be lower as well, but this is not the case.
Quite strange. The signal is somewhere around 80% in general, so I'd be happy with about 300Mbit, still wouldn't understand why it only affects download, and not upload speed... :o
T2U is plugged in a USB3 port, and all USB power saving feature is turned off in windows.
I only have a couple of options for 5GHz, but changing the channel doesn't make any difference, same thing on 36,40,44 and 48. I stll have no idea where's the bottleneck :o