According to Cisco support, the Catalyst 1900 series switches have a combination of some 10 BaseT and some 100BaseT ports. The number of each type of port depends on the specific model you have. It sounds to me like you have the one PC plugged into a 100 BaseT port and the rest are on 10 BaseT ports. Take a look at the manual for your model (or at the router itself) as spidey mentioned and make sure that you plug your PCs into the faster ports on the switch.