I am set up for Gigabit internet and previously saw those speeds with the Xfinity XFi Gateway but... (coincidence?) since I replaced that with my own modem(s), I'm seeing only ~90mbps down on average. I initially noticed this on my SB8200 but have been putting it off for the past 9+ months and recently thought it was related to the modem possibly so I swapped that out for a Netgear CM1200, but get same results.
My home is less than 2 years old, cable drop from the pedestal to the outside groundblock is roughly 15' where it hits the house and is a simple ground. From there it runs to a smart closet/panel where it feeds into an amplifier that is fed to all the outlets directly. Many years ago I worked in the cable industry as a field tech and normally we would try to avoid amplifying the outlet feeding the modem, instead you would split that off directly before the amplifier just so the modem was on a dedicated run. I've yet to try that yet as I'll need to get a good splitter.
In the interim, I opened my modem management page and have included some screenshots. Are these power and SNR levels considered good or bad?