This speed increase comes from when each connection re-transmitts every packet it receives on all of it's ports. An ethernet switch learns the MAC address of an IP address it is connected to and retransmitts packets for known IP addresses only on the corresponding port.
Most Switch hardware is also of greater quality, reliability, and overall speed. They are usually 100 megabit-enabled and, increasingly, gigabit.
On networks with 50+ computers, a good ratio of switches vs. hubs is necessary to prevent individual clients from a bandwidth bottlenecks. Network admins must observe high-traffic areas of their network and increase the use of switches in areas where there is higher traffic. Of couse, all-switched networks are preferable but may be prohibitively expensive.