If a pitcher has a 15-20mph difference in her change up from her other pitches, it's not going to matter if the batter was late on her swing. She's going to swing early or no swing at all because she's fooled...IMO. This depends on the pitcher also, how good is she.
Is a batter way of the plate or on top the plate, overly erect, leaning over, hands high or hands low? All these need considered when calling pitches.