Assuming this is wired direct to the router, have you tried a different ethernet cable?
Yes, of course. There are two cables from the router to my PC (aside from those built in to the walls): One from the router to a patch panel, one from an RJ45 socket to the PC. I've tried switching both of them, and this is where it gets properly interesting:
After spotting this issue, I tried switching a cable. No luck. I tried switching the second cable, things worked fine, but it's too short to have the PC where I want it to be. I plug the old ones back while planning my shopping trip for (a) new cable(s). It still works fine. I'm pretty puzzled by this and I decide to switch back to the first combination of cables that was working. It is no longer working. I tried back and forth with 4 different ethernet cables, and I've come to the conclusion that they're all innocent. I now have the first 2 cables plugged in (the ones that were used when I first spotted this issue) and things are working fine. If I go to the patch panel, pull a cable out and plug it back in, it sometimes works swimmingly and sometimes with the limited speed. In other words: I'm properly confused.
After checking another cable, you should also make sure that your network is secure so you don't have any wifi thieves.
I would also suggest double checking your ISP by calling tech support and seeing if their network shows your connection correctly. I have had to call my ISP a few times over the past few years for messing up my connection speeds and just general overloading too many customers on the same networks. Each time it would take a few hours for them to fix the issue, but eventually they fixed it on their end.
So normally after verifying my speedtests and connections are fine with a few devices but my speeds are not right, it ends up being the ISPs fault.
One of the first thing I did was checking the router for connected devices, I found nothing out of the ordinary. I had a few issues with messed up routing before, so I figured the ISP could've messed this one up to. But before I got around to calling them what I wrote above happened.
Thanks for the help, both of you. It's no longer an issue, but if you do manage to figure something out in regards to what's going on I'm interested. Cheers!