MOSFET gate switching time
It isn't clear whether you mean switcher frequency or switch delay.

In switching converters, each time you switch the gate, and as the MOSFET transitions from being either open or shorted to the opposite state, it briefly becomes resistive.  In this state, the MOSFET consumes power.  Additionally, for each state transition, the gate driver must pump current into or out of the gate capacitance further eroding efficiency. So in general, for the most efficient conversion, you want a lower frequency of operation.

To minimize losses during the resistive phase, the gate driver should have a low output impedance and be able to charge the gate capacitance rapidly. But as you correctly point out, a high slew rate to charge and discharge the gate capacitance can lead to higher EMI. Layout is critical to minimize radiation.

In general, the switch frequency is chosen as a tradeoff between highest efficiency (lower frequency) and the size of the inductor (higher frequency == smaller inductor).  Higher frequency switchers may be easier to filter and should allow smaller capacitive filter elements to achieve the same level of noise and ripple.

You would typically choose the boost controller, MOSFET, inductor and switch frequency based on a combination of the physical size constraints and the desired efficiency. The data sheet should provide enough information to help you make a balanced choice..