Help troubleshooting 1200 internet with a fraction of that actually being delivered.

I am using an Arris T25 modem, on the approved list from Xfinity. It is DOCSIS 3.1 and should be able to deliver the speed.

Yes, I've rebooted the modem. Rebooted my Wifi. In fact I recently updrage my Wifi to a Wifi6E AP and the performance hasn't improved so it must be something on the Comcast end.

I have seen speed tests up to 600Mbps at times, but it is usually 200-400Mbps.