I would agree with you that it's the speed that energy is transferred along the rod. But if I were explaining it to someone who couldn't quite grasp the mechanics of it all, I would tell them to visualize about ten golf balls strung out and attached to each other by a two inch piece of rubber band. Now even though the golf balls are attached to each other, when you pull that first ball the others won't move right away. That's about how it would work with that long rod stretching to outer space.