I recently made the move to install and activate a new modem. I replaced my xfinity modem/router unit with a CM1200 from Netgear and a AX11000 from Asus, unfortunately, after activation my internet speed dropped from roughly 800 mbps to 4.5 mbps.
Both the router and modem are compatible with xfinity and and perform well above 1 gbps, but even after resetting the modem dozens of times and even exchanging it for the Arris SB8200 (which can also handle speeds of over 1 gbps and is compatible with xfinity) my internet speeds have not changed. I've tried bypassing the router and connecting directly into the LAN ports on each modem, but have not managed to see any notable change in speed.
A technician inspected my signal levels and my signals were perfectly in line.