@Tiktaalik 

Given you were able to narrow it to the computer, try the following:

-Disable or remove any security software that the Windows computer may have. That has shown in the past affecting speeds.

-Try all the USB ports available on the computer as the one being used may have issues.

-Make sure you are using the latest driver by visiting our official website.

@Tony 

Good ideas. 

Security Software:

- I don't have any other than Windows Defender. Tried disabling the virus protection and firewall protection. No change. Today particularily bad and getting ~50 Mbps.

Regarding any other software, at this point I've shut down everything that's running in the toolbar. 

USB

- unfortunately one of the first things I tried. I've tried a variety of ports both front and back. 

Driver

No change.

I had already updated the driver as one of the first things I tried, but decided this time to try going to DeviceManager, uninstalling the device, THEN installing the driver. No difference. I downloaded Archer T2U Nano(UN)_V1_190527_Win10 from this link. https://www.tp-link.com/us/support/download/archer-t2u-nano/#Driver. I assume this the most up to date driver? 

The router I have seems to have three access points, but only shows one to the end user.

When I connect on my Mac and option-click on the wifi signal to look at the settings, I can see that I've connected to the 802.11ac connection on channel 52 (5Ghz, 80 Mhz). The Tx Rate is 585Mbps. 

When I do the same on my PC I can see through the Hardware Properties of the connection that I've connected to the Channel 1, 2.4Ghz 802.11n signal and the speed I can view from the Network Connections is 72.2Mbps. 

Using WifiInfoView I can view the details of the 5ghz 802.11 n/ac connection that my Mac connects to. WifiInfoView deems the average signal quality at ~74 though in the task tray the wifi signal only has 2 full lines instead of 3. Using this app I can force a connection to the same 5ghz access point but when I do this I get incredibly bad speeds ~ 2.0Mbps.

For some odd reason, it seem like on this PC the adaptor is really unable to sort out the available 5Gz connection that the Mac is able to use. The odd thing is that when I use this same adaptor on the Mac it works fine and just as well as the built in Mac wifi. 

So is there some issue with the driver on the PC?

@Tiktaalik 

I think you found it, the adapter keeps connecting to the 2.4Ghz while others are on the 5GHz. 

Try and see if you can modify the properties of the adapter to make it connect to the 5GHz instead.

I do not have access to a T2U right now, but see if that option may be there (network connections, right-click the connection, click properties, click on configure, go to advanced) > :

@Tony 

Sadly the problem remains that for some odd reason, even when forced onto 5Ghz, the speed is insanely bad, worse than when on 2.4Ghz. 

I don't have your option you have there to set a 'preferred' band, but I do have an option in that config menu to force it to n/ac. When I force it to n/ac the speed is sub 20 Mbps. 

If it weren't for the fact that while on the mac the adaptor can easily connect to 5Ghz and gets great speeds, I'd think I had a busted adaptor. Instead I have to assume there's some odd interplay with Windows or the PC hardware going on here. 

@Tiktaalik 

With our Deco mesh systems, you can create a guest network and disable either band. Are you able to do that where you make a new wireless network name, and disable the 2.4GHz?

@Tony 

I did try disabling 2.4ghz on the router but yeah no impact. It's not a great fix anyway since I have devices that I want to use which can only use the 2.4ghz band.

@Tiktaalik have you resolved this? Im in the same boat

@EdKi I've had six months of this problem. Last week, I installed an older version of Windows 10 from a USB I found. WiFi was back to consistently high speed. After letting Windows install updates after blocking them for a week, the problem returned. My download speed is measured in kilobytes per second. Definitely a Windows problem. 

Did you find a solution?