I'd start with the ethernet cable you're connecting from your modem/router to the PC. If possible, connect another device (PS4, laptop) with the same cable and check speeds. Also, check to be sure the cable is Cat5e, and not Cat5, as that can and will inhibit speeds over 100 Mbps. If the cable is good and other connected devices are in the normal speed range for the plan, then it's the PC.
If you determine it's the PC, Here is a link to a guide to checking and adjusting the network adapter.