I'm with you - it could certainly be your router, but I think you need to spend a little time narrowing things down between other devices.
Though as Ant says, could easily simply be your internet speed. That said, to be fair, day to day internet use doesn't use quite as much as you'd think. I know of places with 500+ machines using 10mb - 20mb with no drama. But then it does depend a lot on the quality of your connection, contention rates etc etc.