This 'retard the wad' urban myth is, well, retarded.

Think about it.

There's positive pressure all the way up the barrel, even at exit.

The pressure is behind the wad, and the whole payload is accelerating right up to muzzle exit.

How can you 'retard' (slow down) the wad and still accelerate the shot?

Choke effect is venturi effect. As measured by a chronograph full choke velocity of the leading pellets is faster than cylinder bore due to choke effect and not some impossible retardation of the wad.

"The price of good shotgunnery is constant practice" - Fred Kimble