I'm not sure about other carriers, but at United they never changed the 24 minute turn time if you were delayed when they instituted the check bag fees. Carry ons went through the roof and the turn time never changed. You get rid of half or more of the airports that used to have mainline working the ramp and what you get is an almost impossible turn time because of the need to load and unload all of the carry on bags by slow outsourced rampers. But what does mainline care? Every late departure (usually 1-3 minutes) adds up to them not having to pay regional carriers more because of better performance.