does the thickness of the board for a speedball affect the way it bounces?

well a thinner board would naturally have more give, less speed as a result. A thicker board will yield less to the impact than that, thus adding speed because the force isn’t absorbed and is transferred back to the ball more efficiently. Think of it as a water balloon hitting thin ice, versus thick ice. It’s more likely to bounce off of the thicker ice, and the thin ice may even break.