This is the Servo Shop reply, and the case can now be closed:
"I have just tested some of the Tower Pro MG90D 360' servos here, and they all operate in the same manner. They do not have a proportional speed, only a constant speed in either direction of rotation. There is a large "Deadband" area in the centre, where on some occasions you can get a jittering effect from the servos.
I hope this helps,
Thanks and best regards
Robin
Sales at SWM Ltd"