I've never seen such an option before on a router.
I would try and find what is causing the speed of the internet to go so slow. Even with many computers connected simultaneously, it should still be fast as long as people are just web browsing etc.
I suspect that other people on the network are using bandwidth intensive applications, perhaps P2P such as Limewire. If so ask them to refrain from leaving it connected and if they must use it then ask them to use it while others aren't using the internet. If this is the problem and they refuse to listen, you could always block the port on the router that Limewire uses.
Another recommendation would be to run virus and malware scans on all connected computers - some may be inadvertently sending out thousands of spam emails every hour they're connected.