@catapult 

Hi,

There are various possible causes for such an issue.

Personally, the first thing I would check is whether the T3U Plus actually operates in USB 3.x (SuperSpeed) mode. That can be determined by using some software designed to show detailed system information.
In combination with certain hardware these USB dongles sometimes fail to switch to USB 3.x mode and operate in USB 2.0 mode instead, even if they are plugged into an USB 3.x port.

  @woozle USB 3 performs very well. I noticed I'm on wifi6 now: could it be the reason of the issue?

  @catapult 

Wi-Fi 6 routers do still provide full support for previous Wi-Fi standards. That shouldn't be the reason for the low speed.

If you have confirmed with certainty that the Archer T3U Plus indeed switches itself to USB 3 SuperSpeed mode (btw, which software did you use to check that?), then maybe the Wi-Fi channels your router is broadcasting on are too crowded (i.e. also used by other nearby Wi-Fi routers). Could you maybe check that? There is free Wi-Fi scanner/Wi-Fi analyzer software and apps available to do that. If you need I could recommend a couple of them.

  @woozle 

(removed previous reply)

There aren't many other wifi devices around (just 3 o 4), but if you have any recommendation for testing...

[UPDATE]: I casually found my USB 3.0 ports are NOT working at SuperSpeed: it's simply Windows telling me it in the settings panel ("Device can perform faster when connected to USB 3.0"). The fact is that this is one of two SuperSpeed usb ports. Maybe I could change this from the BIOS?

[UPDATE 2]: I have another USB-3 device (a simple Verbatim external hard drive): if I plug it in the same USB3 ports, Windows doesn't display the error message (or warning). So, I suppose is not my USB3 ports not working or a driver issue: it's a problem with the Archer and USB3-SS.

  @catapult 

Yeah, like I indicated in my earlier post certain USB Wi-Fi dongles (to be specific, those that use chips made by the company Realtek) have problems working in USB 3 SuperSpeed mode in combination with certain USB 3 controller chips used in computers.

The way Realtek implemented the USB in these Wi-Fi chips appears to be somewhat unusual. Unlike other USB 3 devices which work in SuperSpeed mode right after plugging them in these USB Wi-Fi dongle chips first start up in USB 2.0 High-Speed mode. After the Wi-Fi dongle has connected to a Wi-Fi network the user can suddenly hear the Windows "disconnect" and then "connect" sound being played and the Wi-Fi network disconnects momentarily. This is when the Wi-Fi dongle is supposed to switch to USB 3 SuperSpeed mode. However, this does not work on every computer.

According to my own experience USB 3.x Wi-Fi dongles using these Realtek Wi-Fi chips work really good with Intel USB 3.x controllers, but not good with AMD USB 3.x controllers. 

Below I added two sample pictures of the UsbTreeView software you suggested (Thanks for that!) that show both conditions. The first screenshot shows my unbranded USB Wi-Fi dongle that uses the same Realtek chip as your Archer T3U connected to the computer's AMD USB controller running only in USB 2.x High-Speed mode and the other screenshot shows the same USB Wi-Fi dongle connected to the Renesas (NEC) USB controller that is built into the same computer.  

  @woozle no AMD here, my pc is Intel-based. Problem now is the following: I should replace this adapter, I've seen another one by Tp-link, but I'm afraid it may reproduce the same issue.

  @catapult 

If your PC uses Intel hardware, then there is still hope. I remember with very old drivers it didn't even work in SuperSpeed mode with Intel USB 3 controllers.

There is a thread in this forum where a newer "Realtek chipset driver" is offered for download. https://community.tp-link.com/en/home/forum/topic/509006

The Archer T3U Plus model is listed in the post, but I am not sure if the driver will actualy automatically install for this model if its USB ID is indeed 2357:0138 as mentioned on the Internet. So, after running the driver installer you need to verify in the device manager whether the driver version has actually changed to 1030.44.0531.2021. If not, then you would have to force-install the driver via the "Have disk" method.

  @woozle It kinda works, now   :-)   I haven't done anything: that is, yesterday I manually updated the driver, but nothing changed. Now: I have a DELL computer. This computer features an application called "Dell Command | Update". Its main task is automatically updating drivers, BIOS, etc. Half an hour ago, it displayed a notification relative to an audio driver update. I updated the driver and, since I was required to restart, I restarted the pc. Once restarted, I had no WiFi connection anymore and the antenna light was off: I unplugged the antenna e plugged in it again. In the windows settings panel, I found it was now recognized as 3.0 (no more warning messages). Then I started UsbTreeView and found the triplet: Superspeed/Superspeed/Superspeed. At this point, I tested the connection and found the speed doubled (before: 220, now 481: not a whole Gbps, maybe it is impossible to reach 867, but in anyway it's better now)   :-)

  @catapult 

Good to hear that is works now.

Like you say, the rated speeds of Wi-Fi devices are impossible to reach. They are just theoretical (mathematical) values and can be used for comparing different products.

481 Mbps at around 5 meters distance is basically as good as it gets for any "AC1200" or "AC1300" Wi-Fi device.

If faster speeds are needed, then one should upgrade both the Wi-Fi router and the Wi-Fi adapter to Wi-Fi 6 (AX). Anything labeled AX1500 or AX1800 should get to around 600-700 Mbps of actual data throughput on the 5 GHz band.

  @woozle Ok, thanks for your precious comments, time and help   :-)