You can switch a MOSFET at any speed that you'd like...but more precisely, up to the maximum speed as dictated by the MOSFET's parasitic capacitance.
A MOSFET has a few intrinsic capacitances, as byproducts of the way they are constructed. The higher those capacitances, the slower the MOSFET turns on and off, for a given drive signal. As a simple model, think of the gate of a MOSFET as a capacitor (CISS on the datasheet). To change the voltage across that capacitance enough to reach the gate-threshold voltage, it will take some finite amount of time.
Depends what you are doing, but high capacitance MOSFETs can incur big switching losses.