Windows oddly throttling all downloads via wireless connections

We have an HP Elite Slice computer with builtin 802.11ac wireless. Downloads from the Internet are ridiculously slow, like 11 minutes to download a 21 MB file. Interestingly, testing on speedtest.net works normally.

I tested an external USB/wifi adapter and it too is super slow to download files from the Internet.

I further tested a second wireless router and it proved super slow using either the builtin wifi or my usb adapter.

I ran a super long cable to the first wireless router and on there, downloads proceed normally, so it's definitely specific to wifi.

It's as if Windows knows when I am downloading on wifi and throttles the connection immensely.

What could be going on?

Caledon Ken said:

If you go to "Advance Power Mgmt Settings" is Wireless Adapter Settings set to maximum performance?
Also worth checking if you have any interference
inSSIDer 3.1.2.1 Download - TechSpot

Sorry, I forgot to respond. This is set to Maximum Performance. This couldn't be the cause. Remember that it's the specific downloading of files from the web that is slow. Speedtest.net itself gets reasonable download speeds.

Interference is unlikely as other machines in the vicinity connected to this wifi router are not showing this behavior and it wouldn't account for Speedtest's being normal.

The antivirus is the Windows 10 builtin.

We ended up plugging into the wifi router directly.

Are the computer and router in the same room? You will be aware that wifi reception is highly susceptible to being affected both by distance and by intervening walls and floors.

The primary router was several feet away, but the test router was on the same table as the computer. Interference really can't explain how downloads are super slow while speedtest.net is not. That's just weird.