As some of us said when MLB announced the replay system, using "challenges" is a mistake. As outlined above, the way to do this was always to have a fifth ump in a booth upstairs, responsible for reviewing calls in real time, which would, in a high percentage of plays, result in zero noticeable delay. It's only close calls that need to be looked at, and there is often confirmation one way or the other with the 1st or 2nd slow mo replay, which the ump upstairs would have confirmed before the next hitter is even in the box. If he/she needs more time, signal the HP ump. Getting the calls correct is what matters, and they could do that simply, in a majority of cases, quite quickly and easily.