Why it's so slow
Most airlines board the plane from the rear to the front, which in theory should work well. But people aren't exactly boarding in perfect seat order, and when the entire line has to stop every time an old lady can't hoist a 35lb bag into an overhead bin, things slow down considerably.
But it's not like people could magically move through each other in the '70s, so why is it slower now? Baggage fees, for one. Fewer people checking bags means a lot more giant carry-ons, meaning a lot more people clogging up the aisles trying to put them away. It's not the only culprit, but it's a big one.
Spirit, of all airlines, has actually found a handy (if onerous) solution to this: charge for carry-on bags, too. (It charges more, in fact, than it charges for checked bags. And if you have to check a bag at the gate, you can get hit with a $100 fee.) Naturally people become more likely to suck it up and check their bags. The airline claims to board about five minutes faster than its competitors.