Hello. About a month ago I received and e-mail that stated that there would be an upgrade in my area. About a week after that I get another e-mail saying that I am to enjoy the free upgrade that is now finished. Ever since that e-mail my speeds have been cut in half. I used to download 15-20MBps and now I get 6-10MBps. I have the performance plus internet with performance pro thing on top.
Try calling 1-800-comcast and ask a rep to completely remove the modem from your account and to "re-provision" it from scratch. Also ask them to confirm that the provisioning and account data in their database is correct and complete and that the correct configuration / bootfile for your subscribed to speed tier is being pushed to the modem.