Eh, No, it's not the rate or "speed" which they move.

Rather you want to adjust the "endpoints"... or "travel".

That is how FAR the servo turns when you activate the switch. ( or for other channels how far the servo turns when you move the sticks ).

This allows you to have more precise control over the flaps, so flipping the switch can result in a 30% drop in the flaps as opposed to all the way down.