They don’t charge for carry-ons, that’s one possible reason. And they don’t charge for checked bags, which also takes time to load and unload. So Southwest probably has more carry on baggage and checked baggage to load compared to Ryanair. Also WN does not normally deplane from both front and rear exits in most cities (minus a handful of exceptions). Doing so would I’m sure help turn flights faster.
You've almost certainly nailed it with the free checked baggage and single exit points. But Ryanair don't actually charge for carry-ons. Everyone gets to take a bag that will fit underneath the seat in front, and a larger bag too. The larger bag will be gatechecked free of charge unless the passenger has one of several ticket types including the word Plus, or purchases a package called Priority Plus Two Cabin Bags, in which case they can bring it into the cabin with them. The number of Plus tickets or packages sold per flight is restricted to make sure there is enough overhead space, and they can be merciless with oversize/overweight carry-on bags.
In fact the gatechecked carry-on must add to the volume the baggage handlers have to deal with.
And of course Ryanair, having tried SouthWest style free seating for years, has ended up with allocated/reserved seating instead, and finds it no slower to board.