I have this weird problem where when I put a router between my computer (running Lubuntu 24.04) and my modem, I unpredictably get a few extra seconds of latency for some requests, even while I'm still using Ethernet. I notice it mostly for stuff other than HTTP, like getting email over IMAP or connecting to a server with SSH. I'm pretty stumped because (a) I'm a programmer, but know pretty much nothing about low-level networking, and (b) I can't reproduce the problem consistently.

Any ideas on how to start debugging this?

bad cable maybe

Hmm, I'd have to get a new one to check, but could be worth a shot.

check what the ports are negotiating to if you can. I've seen bad cables cause things to negotiate to 100m half duplex. Generally would expect that to cause slower speeds and not necessarily increased latency but who knows

I know my current router has a bug that specifically affects connecting to starlink routers too. It would negotiate down to 10m when plugged into the starlink so they have a beta firmware that fixes that

@Kodiologist not sure. You can check from the OS for the router to computer connection. My router will say the link speed and duplex on the port view webui page