Upload speed is half of what is specified in the plan

Hi, I have the Superfast Plan of 800 down and 35 up, and I have never got the upload speed to what was specified, the max I hit is half of that, around 16 to 17. I have Arris S33 surfboard DOCSIS 3.1, I'm not looking for a next gen speed as my plan limits my upload to 35, and I read the specs that Arris S33 can easily do 35 up, can some one help?