Well I wouldn’t discount it, but I do think it’s effect is minimal. Anytime there’s a runner on base it is a distraction to the pitcher whether he runs or not. Even Joe Mauer would steal a base if he was ignored. Holding runners is part of the game, even slow runners need some attention. I would tend to discount ideas that fast runners disrupt a pitchers game to the extent that he can’t perform. Those cases are rare, not the norm.