A lot of providers say "as fast as xx megabits" where xx is a number. Then you get the service and find the speed is not as fast.
It has nothing to do with your computer, even if it is an old Pentium II with a 10 megabit ethernet card. For cable, the xx megabits is the speed between you and the something called a "head-end". Kind of a fancy word for a large router. If 10 neighbors are on the head-end your speeds will be close to advertized, between you and the head-end, but between you and the web the speed speed is not longer the issue.
But the real reason for the slower speed is not your connection, but the lag in the hops your connection takes. Let's say you connect to yahoo or google. That connection can take 10, 20, and maybe 30 'hops'. Each hop introduces a lag, and each lag slows down the connection. So, unless you are transfering files with your next door neighbor your connection speed is going to be dependent more on who you are conencting to, than on the connection speed.
The same limitation exists with DSL. The only difference is that the 'router' is in the central office (CO) or at a switching station, rather than at a head-end.