Why is higher transconductance relates to faster transistor switch?
Quite often it is the gate-source capacitance of a MOSFET that is the dominant factor on how quickly a MOSFET can be turned on or off. Quite high currents have to be injected into the gate capacitance to quickly change the gate voltage so, if the transconductance is (say) twice as much on MOSFET A than MOSFET B then, to adequately switch a particular load current, you only need to change the gate voltage by half the amount compared to MOSFET B.

This usually results in an increase of switching speed for a given current injected into the gate.