Sounds like you've got a very basic router...might want to try flashing DD-WRT or another third party firmware on it, it would give you more capabilities.
I have, at most, 14 PC's connected to a DLink DIR-655, with stock firmware, coming in over a 3 router wifi/mesh network, and nothing ever slows down like you mention.
EDIT : reading seepy's post...I think you might be hosed, if you're stuck with their router/gateway/etc. maybe get another router in front of that gateway/router/switch that they supplied you with, since you can't remove it from the network?