The way to think about it, is the spike itself is a waveform, and the RC snubber is tuned for that frequency.
Lets say the switching is 100kHz squarewave 50% duty cycle, 10us p-p.
The spike appears on the scope as 0.25us, or 1/20th of the positive part of the main waveform.
f = 1/ time;
= 1 / 0.25us
= 4Mhz
So the spike is 4Mhz, and you design the snubber for that.
Anyway the trade off between faster risetime, and not needing a snubber comes down to:
a. Mosfet is most efficient when either fully on, or fully off, in-between states produce heat as Drain-Source is higher resistance.
b. Slower rise time means less EMI issues, and less expense in snubber components.
c. The voltage overshoot is also an issue for the poor mosfet, as you need to over specify it's voltage to cover any spikes no matter how short.
The practical RC snubber networks I've been looking at lately (flyback SMPS), all have a diode as well, presumably to reduce ringing.