Sorry, but I just need to rant somewhere...
I just realised a Raspberry Pi 4 of mine was only doing 100Mb/s networking from seeing a slow file transfer to my PC. According to ethtool, the gigabit switch it was plugged into would only negotiate up to 100Mb/sec.
I swapped the cable expecting a fault there and all is now well, but it looks like the damned cable only ever had two pairs wired up.
Really, a not fully connected network cable, by design. The missing copper teeth in the RJ45 caught my eye as I removed it. I have no idea what bit of kit that got shipped with or whether it was bought from some Amazon seller or whatever, but the cost of these things is mainly in the assembly and testing so the money saved on a short cable by only using 4 core cat 5e must be tiny. I've seen plenty of junk in my time, but this is new junk to me.