I was trying to figure out slow network speeds one day and I finally swapped an ethernet cable and got full speed.
It turns out some ethernet cables are missing wires - and capped at 100Mbps! Wait what? I didn't even know that was a thing. I don't know where I got the wire, but into the trash it goes.
100 Mbps is sometimes just enough for the job. And it’s easy to see that you only have two pairs instead of four – just look at the connector.