The transient response question is complex, with many inputs.
Power train considerations include:
- the size of the output inductor
- the amount of output capacitance
- the type of output capacitance (ceramic, electrolytic)
Control considerations include:
- the control scheme (voltage mode, current mode, V2)
- the operating frequency
- the operating mode (DCM, BCM, CCM)
The transient response is only indirectly related to switching frequency, in that the switching frequency imposes an upper limit on the maximum closed-loop crossover frequency that's realistically achievable. (Generally, the crossover should be at most 1/4 of the switching frequency.) A 1MHz switching frequency buck with a loop crossover at 100Hz will be just as slow in responding as a 100kHz switching frequency buck with the same 100Hz crossover.
The mode of operation is also important. Operating a buck in DCM (discontinuous conduction mode, where there's a period of zero inductor current per switching cycle) helps damp the output filter response and makes compensation easier (and allows for a higher crossover vs. other modes). If the converter transitions into CCM, the response abruptly changes (you get a large gain peak and an abrupt phase shift) and you generally need a lower crossover frequency to ensure stability (vs. DCM)
Current-mode control can provide better transient response (the inner current loop can generally respond faster than the outer voltage loop) with some penalties (current sensing, slope compensation to avoid subharmonic oscillation, etc.) V2 control can also improve transient response vs. a normal voltage loop, but is complex as well (sensing the capacitor current can be tricky).
I could go on, but I think you get the general idea.
If you're designing the buck yourself, you should model/simulate/iteratively test to get the optimal transient response for your particular converter. If you're buying one from someone, they should have this analysis available for you. You cannot generalize based on switching frequency.