Hello  @Xsile 

Thank you for your detailed post, you could also try some tips here if 5Ghz speed is not ideal: 

1. You could also run a speed test on a different network device like Laptop/ mobile phones at the same location of the PC which installed T4UH adapter.

Confirm other network device can get good 5Ghz Wi-Fi speed at the same location 
2. Log in router and try a different 5Ghz channel like 36/40/44/48 etc. and set 5Ghz channel width at 80Mhz. 
3.  Disable the built-in adapter when use TP-Link adapter 
4.  Try this latest chipset driver for and see if it makes any difference 

If still the same case,  can you provide some information here to help investigate further 

1. a photo of Speedtest result using TP-link adapter & a photo of Wireless link speed

2. a photo of Speedtest on a different network device & Wi-Fi link speed (if it has)
3. the model of router 
Please help check the driver version of adapter and computer motherboard info and send us the photos: https://www.tp-link.com/support/faq/2722/

(Skip if you've provided) Thank you.