The slappers with some speed and the best bat control will survive. Not necessarily the fastest. Smart helps, too.
The key to being an effective slapper is to be able to read the defense like an NFL QB does pre-snap. Then, she has to have enough bat control to be able to drag bunt, power slap, half-slap or swing away based on what the defense is doing against her. A skilled slapper who can do these things will be more effective than a less-skilled slapper who is faster, simply because she is hitting 'em where they ain't.
Speed does matter, though. I don't look to convert girls unless they're fast .... usually in the top 20%. They have to be able to beat out a drag bunt when the D is in tight and they also need to be able to beat out a slap that isn't charged hard enough by an infielder. That is a huge part of their game and the main reasons why you don't see many slow slappers.
LAS nailed it here. There is always a hole in the defense. They have to be able to exploit it.
If your slapper can hit away too, it makes it impossible to defend against her.