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.
About eight months ago, I picked up a Dell Latitude C400 from thrift shop, planning to put some sort of Linux on it. The laptop booted, but I got the gray screen of death (later diagnosed as a hard drive lock).