Engine won't idle down

I would bring it up to the dealer.

I checked customer reviews for your vehicle and didn't find the same complaint. One complained about rough idle and another complained about slow throttle response. I checked Technical Service Bulletins and didn't find one either. Your truck is new so it's hard to find similar problems with other owners. With that said, in my opinion, that is not supposed to happen. If it takes a full 3 seconds to come down that is a problem. Do you mean that when you push in the clutch pedal the engine stays at whatever rpm's it was at for 3 seconds? First, I would call another Ford dealer and describe your problem, without saying you already talked to another dealer and see what they say. Then either take it to that dealer if they say they'll take a look at it or go to the original dealer and raise some hell.