while you should get more then 30mbps, you certainly need a gigabit switch for a 120mbps internet connection.
The problem (what is causing only 30mbps) is going to either be the current switch or there is possibly a badly terminated wire going between the router and switch. Do you have a cable tester?
Now, to answer the literal question in your title is that you don't.
If you want all ports to be capable of receiving your max internet connection then get a gigabit switch
If you want all ports to get an equal amount of internet speed under simultaneous load, you need a managed (or semi-managed) gigabit switch that has QoS.