Vero4k+ Extremely slow network speed

There is no mentionable context. Test 2 is just a repeat of test 1. As I stated previously, the speed varies greatly, but is shit most of the time. The changes are 100% random and non-reproducible.
As I said, with a 100MBit cable the connection is rock solid 100MBit all the time.

Understood. A bit of a long shot, but if your system isn’t too highly configured, it might be worth going for a reinstall from a fresh image.

Your iperf3 tests are showing poor performance in both directions, so that’s different from the problem with the original batch of V4K+s, which had a problem on the TX side. As already mentioned, it’s also not a duplex mismatch.

If the resinstall doesn’t work, the odds are it’s probably a hardware problem, eg an intermittently failing chip or possibly a dry joint. If 100 Mbits is sufficient, then fine, stay with the Cat5 cable. If you need more, you still have the option of a gigabit USB ethernet adapter, though it won’t give anywhere near full gigabit speeds.

You could also consider cleaning the Ethernet port.