"Very different" download speed on two computers connected to the same switch, what should I do?

Question

Computer networks

At the dacha there is a small grid: a 4G antenna connected to a switch, and wires from it to different rooms for computers, printers and WiFi routers. Recently, there was no antenna yet, and there was a modem, but then we got fed up with such speed and bought an antenna. The first couple of days everything was fine, the speed was excellent, the flight was normal, but after a couple of days on one of the computers the download speed suddenly began to sag, and on the other computers everything was ok. And to sag not by a megabit or two, but noticeably, does not rise above half a megabit per second. With the download speed, everything looks great, on the speedtest the result is (yesterday's). What could be the problem? The cable is crimped normally, checked with a tester, Kasperych is protected from viruses (crystal). On other computers, the download speed is ~10 mbps.
PS I am far from being a professional in such things, I may not know things that are obvious from your point of view, please take this into account when answering.

Answer 1

Many thanks to throughtheether for the help and advice! Having dragged the system unit from another room, I found out that everything was ok with the wire. After scanning the computer with all known antiviruses and examining all running processes, I found out that the problem is also not in viruses. There was only hardware left, so I bought a new network adapter (before that, the cable was inserted into the built-in network card of the motherboard), and voila, it all worked !

Answer 2

but after a couple of days on one of the computers, the download speed suddenly began to sag, and on the other computers everything was ok.

How (wiring diagram, etc.) is the problem computer different from the rest?
Connect a laptop instead of the problem computer with the same cable and observe the changes. If there is no change, try connecting to a different switch port.
So far, it is rather difficult to judge the cause of the problem.