I had a similar experience with Comast/Xfinity. Used my own Arris Surfboard with their service and never got the speeds they promised - that I was paying for.
When I upgraded to a better Arris Surfboard (3.1 DOCSIS) the problems remained.
Switched to Verizon and have been getting speeds promised ever since. Verizon Ethernet Gateway solved all issues I had.
No one should ever trust Comcast, and although I have had good service, Verizon is also highly suspect. For my location, those are the only two options. Using your own equipment with those two providers is a way for those providers to dismiss your complaints.