What is the importance gate capacitance in CMOS device?

For the gate to change state, the gate capacitance must be charged or discharged. Since the transistor driving the gate has a certain amount of output impedance (resistance), this together with the gate capacitance forms an RC network. The gate capacitance must charge through the driver's output impedance, and this takes time. So, gate capacitance limits the maximum speed at which the device can be operated. Decrease the capacitance, and you can clock the device faster!