S33 model is compatible with the xfinity speed plan so you can try chancing the device to check the speed on your modem. since this is just a cable modem you can check the optimal speed of your modem by connecting device to the modem directly without router. If still slow speed issue persist we need to check the cable signal level on the modem interface so just follow the steps mentioned on the below article to work on the case further.

http://arris.force.com/consumers/articles/General_FAQs/S33-Cable-Signal-Levels

Thanks everyone so the tech came out and ran a test on the line and showed me it's all normal. But it had alil high of something he put something on the end of the line and said I might need a new modem.. all I can say is wow