The last layer of pellets would probably ride the wad out the muzzle. But, the front layer has had higher velocity for a small fraction of a second. Whether or not the last layer of pellets has full acceleration from the choke, the shot string will be stretched out due to the front layer having longer at higher velocity. I don't think anything slows down. The acceleration of the pellets is provided by a drop in pressure internal to the shot column. I know that is counter-intuitive, but it is true.

Does that help? More questions or observations?

DDA