Paying for 900 mbps, only getting 200 mbps max.

Hello @user_b0c335 That sounds like a terribly slow speed to have with our 900 Mbps service. Since you have done all the hard work and isolated to a single device hardwired (Thank you!) and still seeing the slow speeds we would need to look at hardware and wiring. Are you using a CAT 6 ethernet cable? Is your computer compatible with 900+ Mbps speeds? Are you using a modem/router gateway devices leased from Xfinity? I can run a few checks to make sure the right service is being delivered to the modem and we can poke around from there to see what is causing the slow speeds. 

I am using the Xfi Gateway with CAT7 cables and I have a motherboard with a gigabit compatible network port

Excellent! Thank you for confirming both items! With everything you have check and the issue ongoing, let's take a look at the modem and what speeds are being received. If you could please send me a direct message with your name and service address.

To send a direct message, please click on the chat icon in the top-right corner of the screen, and select "Xfinity Support" to initiate a live chat. 
Click "Sign In" if necessary
Click the "direct messaging" icon or https://comca.st/3J0ir1l
Click the "New message" (pencil and paper) icon
Type "Xfinity Support" in the "To:" line and select "Xfinity Support" from the drop-down list which appears. The "Xfinity Support" graphic replaces the "To:" line
Type your message in the text area near the bottom of the window
Press Enter to send it

Do you need anymore information?