Of course the speed is controlled else where, but the cable needs to handle all the traffic. A Cat 3 cable will definitely limit performance if you have a 10mbit/s connection or more. The cable is a highway, sure you can have lots of cars wanting to go through it, but if it is only 1 lane is would be very slow. Newer cables also have better shielding from things such as crosstalk. A cat 5e cable should be the cheapest you consider, its also really all that is necesssary. And there will not be a noticeable impact from getting a 50ft cable.