I recently changed my xFinity Internet plan from 300mbps to 1000mbps. This involves some changes to the box outside the house and a new xFinity modem. I've never had this bad of Internet quality in my house. When my kids are playing online, the video streaming becomes annoying with the buffering.
The temporary fix is to periodically restart the modem. When I had my 300mbps, I was using my own modem (compatible with Comcast) and never had any issues. I've narrowed down my Internet issues to the modem. I'm replacing it with an Arris SB82xx.