Update: The issue is now resolved. It turned out that my switch was starting to fail — although link lights were on, it silently dropped back to 100Mbps. Today, the switch completely died, and after replacing it with a new Gigabit switch, the LAN speed immediately returned to normal (~900 Mbps via iperf3).
Thanks to everyone who read or considered the issue. Hopefully this helps someone else troubleshooting similar symptoms.
Thanks to everyone who read or considered the issue. Hopefully this helps someone else troubleshooting similar symptoms.