Why is my wireless LAN suddenly transferring files so slowly?

[Kyuu]

If the link below describes how I should check this on both machines, I will do that and report back. I know that nothing has changed in terms of hardware. All I can think is that somehow the wireless spectrum nearby is a lot more crowded/noisy suddenly.

https://www.tp-link.com/us/support/faq/2265/

Yes that would be the link speed (PHY rate).

https://kb.netgear.com/19668/WiFi-Link-Rate-and-Transfer-Speed

 

[Kyuu]

But after all is said and done I feel totally silly, because it turns out the stupid thing had at some point connected automatically to the 2.4 GHz band and not to the 5 GHz one. I corrected that and configured it so it wouldn't connect automatically to the 2.4 GHz band.

Yeah I was about to say, that 144.4 Mbps PHY rate rang a bell with me, which I then quickly confirmed:

Exactly the PHY rate one would expect with a 2x2 antenna on 20 MHz channels, which is what one would expect to see on a (good) 2.4 GHz connection.