They claim they don't - see https://www.cnet.com/news/comcast-says-it-isnt-throttling-heavy-internet-users-anymore.
You might want to check Connection Troubleshooting Tips and, if you still need help, Information Requested for Connection-Related Posts.
i know for a fact that they do..i have 1gig net. i should NOT be buffiering at all. and when i check my net speeds it is where it should be.
I have the Xfinity 1GB service. It is not throttled.
Thanks for posting to the Xfinity Forums. @RobertWy is correct. You speed issues may be related to other issues. Can you please post your signals here so we can try to help isolate a cause for the speed issues like premise signal levels or issues at the local node? Also, are you using WiFi or connected using an Ethernet cord? We can guarantee the same speeds over WiFi that you would get hard lined. There are several environmental factors that could interfere and cause buffering that you are experiencing.