I've been on a SW trip with no plane change, and just barely had time after the head count to get to my bag above in the bin, get something out of it, put it back, and then new people started to get on board.
Of course the advantage to this for you is that you get the best seats as you can move anywhere you want once they count you. (or even before if it's a short and quick move). You'll have even better seat options than the new flyers who paid business class. It's not really fair, but that's SW.
FYI, other airlines can also turn planes around quickly if they need to, but SW's schedule demands that they do it all the time to squeeze out higher profits from more flights per day. So, it's routine for them to do it fast.