swing. If you're saying players with a high average bat speed would be less successful slowing their bat down to below the threshold - you're probably right. However, that's also true now. Players with a high bat speed would probably have their bat travel further when trying to stop it, and would be more likely to have a 1st/3rd base ump call it a swing on appeal.

The current system is the umps judge the "intent" to swing. I don't know of a better way to convert "intent" to numerical data than the speed of the bat as it crosses the plate. The higher that speed, the more intent there was to try to hit the ball.