Network performance deteriorates when uninvolved device on switch is shut down

However, as soon as I turn off the nettop which isn't involved in the measurement process at all, I get a lousy performance of roughly 3.5 MB/s (SSH) or 58 Mbits/s (iperf).

This strongly indicates that the switch (which is a no-brand so-so specimen) drops the entire backplane to 100mbit when you disconnect the nettop.

I agree it is worse than weird - it's unacceptable.

But there you have it.

It's exactly as mfarver suggested: My switch is broken. I replaced it with a new one (a Cisco SG-200-08 this time) and now everything works like a charm, regardless of which additional devices are connected and which aren't.