Slow upload speed on machine running Ubuntu Server 18.04

I’m currently running Ubuntu Server 18.04 on a home server build to act as a Plex server, among other things. I have it connected via ethernet (with a 5-port switch between it and the router), and get download speeds consistently in the 750-900 MBit/s range when running speedtest-cli, which is expected. However, the upload speed has never gone above 5 MBit/s, despite every other machine in the house getting speeds around 300-400 or more when performing speedtests.

How can I diagnose what might be causing these slow upload speeds? I’m unsure what might be causing the issue if other devices are having no trouble on the same network, including another computer that is connected to the same 5-port switch. I tried basic things like resetting the router and swapping cables and ports on the switch but that did not change anything.