it's not that mci's security is necessarily faster, it's that time from dropoff to sitting down at your gate is faster.
midway is fast enough at getting you through security. thing is it can be a 5-10 minute walk once you're through security. that's not including the time it takes to get from the train to the security checkpoint, which is about another 5 minutes.
let me put it this way, if you're running late getting to the airport trying to catch a flight, you're in much better shape if the airport is MCI than if it's MDW.