This is determined by the model. The best water softener shower heads are designed with this in mind, with flow rates of 2.5 gallons per minute, which is the legally mandated rate for showers. A machine rated at 2.0 gallons per minute isn't too bad either, and you won't notice much of a difference. Anything less than this, on the other hand, can have a significant impact on your showering experience, especially if you reside in a region with extremely hard water.