Router On A Stick
Is a technique to route between different VLAN (Virtual Local Area Network).
See VLAN (Virtual Local Area Network) > How to communicate between different VLAN for its limitation.
It basically used VLAN Trunking to connect to the router so that the VLAN can talk to eachother via the same trunk
However you will be limited by the cable speed. Assume the cable speed is 10 Mbps, all the VLAN will be shared with the same limit of 10 Mbps