I find the rear defroster super slow, to the point that I made an appointment for service. Is this something "normal" for the MDX?
In my old Pilot and my wife's RX the defroster the change was almost instant.

 

I used it yesterday and saw that it started working a few seconds after turning it on. It could be colder than usually inside your cabin or your engine hasn't warmed up. Are you defrosting after the car has been sitting for a while?

 

Mine works a little slow also, but it does the job. Considering the electrical loads and the crappy battery, maybe it's a good thing that it isn't super fast.

 

Electrical current going through resistance wires should generate heat regardless of the engine temperature.
Yes, I am trying to defrost after the car is sitting in cold damp weather. Sometimes I get to my destination and I yet have to see a defrosted rear window.