Except that you're forgetting that the average plane, most any plane that's predominantly coach (ie, anything except all-business or all-first planes) is full of infrequent flyers who fool around at least as much with figuring out where their assigned seat is (especially in summer, around holidays, etc).

(These overlap a lot with the tons of people in Group 3 or 5 who try to board with first-class, because they have no clue about boarding order, don't listen carefully to boarding annoucements, etc.)

So on Southwest they can just grab a seat without having to know its number or letter. On other airlines it's slower for them (and thus the people behind them), because for reason it takes them forever just to figure out how seat numbers work!

If people board Southwest so inefficiently, how are they able to get the 25-minute turn that other airlines can't seem to manage with same-size planes?

Yeah, isn't it funny that people who come to the Southwest board to complain about the lack of assigned seating and offer theories about why it's so bad are both ones who rarely fly Southwest and ones who wouldn't fly Southwest even if they changed this policy!