Were you using firewall software on your computer? If not then maybe you were letting in a bunch of traffic that the router's internal firewall has now put a stop to. Can't think of another reason why a router would speed things up really.
- As wmatistic suggested, try a known good monitor. That inlcudes hooking up your TV via s-video cable. If the problem exsists on more than one monitor, you're looking at a possible video card problem.