Agreed. Let the service department help you out or your sales person who should have explained it to you.
I've had this on my BMW's for years and I'll tell you some work differently than others. My previous 2013 X5 worked perfect, light rain, slow steady cleaning of the windshield, heavy rain and it would speed up.
My current 2015 X5M is all kinds of odd... It will be misting out and it's going crazy fast, but then slows down and vice versa.
From what I know of these, they use an infrared sensor behind the mirror (not user adjustable) and sense the water on the windshield as well as the speed of the vehicle.
I know through coding (an entirely different story where you can modify the factory settings through changing the coded settings) you can make it more or less sensitive, but this should work 'normally' from the dealer.
So as mentioned above, the best advice is take it back to the dealer and explain it to them so they can check it out and help you.