Our minimum turn time on a B737 is 30 minutes. B727 is about 40 minutes. B757 are given 45 minutes. A few reasons that WN is able to pull off quicker turns are :
-No meals to load
-No Interline bags to deal with
-No Pre-boarding for First Class
-All their gates are in close proximity, so minimal time is needed to transfer connecting bags.
-One aircraft type, so only one type of aircraft to learn for employees.
-No-Hub and Spoke System to louse things up
-High employee morale