@MX46 

I assume you are connected via an Ethernet cable, not wireless?

Are you checking the speed using the same cable, that is the same one from the PC to the router and to the Modem?

When you change devicess connected to the Modem, do you power off all devices and then power on the modem first and then the device connected to it?

Assume you have only used wired connections, have you tests ALL router Ports?

Have you tried a different cable?

Did you have a different router before and got full speed?

@IrvSp 

It's slow regardless if it's wired or wireless. I've tried multiple cat7 cables. I've tried other routers and consistently get better speeds. Also tested every single port. Same cables used for testing directly to the cable modem and achieving full speed.

@MX46 

You could try uninstalling Tether then testing the speeds again. The QoS did slow down speeds for some. Another thing is if you notice the router getting very warm try repositioning it for better airflow.

@Tony 

I've unbound the router from remote management and uninstalled Tether. However, for QoS there is no toggle switch to turn it off. I have it set to Standard and turned off any device priorities. The router is also well ventilated. 

That improved the speed a little bit. I'm hitting around between 550 - 600Mbps. Still slower than full duplex but better than what it was before.