I had a lot of trouble with this as a young child - in fact, I couldn't stand to play slowly, or to listen to anything slow, for that matter. If listening to an orchestra concert I couldn't wait until the pace picked up again. As for generating a slow tempo, I simply couldn't do it.
Therefore, aware of this trap, I draw the attention of my young students to the magic of vibration: Once involved with that, they are in no hurry to move rapidly. In fact, when rapid movement is called for they have no difficulty playing "under tempo" as that is where their ears are most engaged. Playing at tempo too soon is all too often simply a matter of drill with no ear involvement.