I'm not sure what you are asking. Do you want to slow down the standard formula that comes with that deformer? Just write a new formula, that's the whole point, or in a pinch just do a division at the end of the formula:
This is what comes in mine when I apply it: Sin((u+t)*2.0*PI)*0.2. So you can just do this: Sin((u+t)*2.0*PI)*0.2/2 to cut the effect in half (or just get rid of the *2 in the middle of the equation which amounts to the same thing). And that "divide by" can be anything you like except zero.