Ethernet speed stuck at 90 - 100 Mbps is usually caused by a hardware problem:
- Faulty, loose or poor quality network cable.
- Faulty Ethernet cable connector.
- Faulty Ethernet LAN wall socket.
- Faulty Ethernet LAN socket on PC or router/switch.
The first test to do is to unplug/replug the network cable at both ends or try replacing the cable with a higher quality or different brand.
- Is the PC's network cable plugged into the Ethernet wall socket or is it plugged directly into the router's LAN port?
- Are your PC and router in the same room or in different rooms or on different floors of the house? If you have another computer with Gigabit Ethernet connectivity, unplug the network cable from your PC and plug it into the other computer and check if the speed is regular.