i dont think the length of the coaxial cable matters .. i think thats based on light or something like that..
now a shorter, uncrinkled ethernet cord would help.
uncrinkle the enet cord you have, and if that doesnt do anything, call comcast, and see if they can check the router/cable modem ... if that does check out to be ok, then have them check settings on the computer, and if that checks out ok, then its the comcast lines, and you need to figure out what speed you are paying for, and then complain about not getting what your paying for, and then if that doesnt do anything, switch if you can to a different service