> I have trouble getting my computers to copy data between each other anywhere near the theoretical limit for a 1 Gbps NIC, despite being connected through only a single switch.

Maybe your LAN cables have deteriorated?

Reaching 100 MiB/s should be easy peasy for a PC; in my case (2 10Gbps switches) by using nvme SSDs I reach almost 950 MiB/s and with 4x raidz1 HDDs I reach 450-650 MiB/s.

Until a few years ago I was smoking in my flat and often I had to change lan cables (whatever's in the smoke was sticking to the connectors)