The frame rate calc for ppm is so you can determine the max channels some of the systems can handle like autopilots and some heli systems that are limited to 20ms for instance. It is unrelated to servo output speed. Output speed of servos is how fast a single pulse can be modulated (sent) to it. The ppm speed when set to auto will be fast but the entire pulse train for X channels selected will be sent before the pause. So when a receiver is speaking ppm to another receiver we just let them do their thing. But when a servo is also attached to the sat rx then we might have to slow the pwm down. So since there is only one speed setting we have to use that. Th reason it works in auto usually anyway is because the pwm pulse is sent during ppm rest times in one pulse group which isn't extremely fast.
Moral: use auto if the servos operate normally. Then slow it down only if necessary
Hope that makes a little sense and doesn't confuse peeps