To all the people telling OP they're wrong, you don't fly enough. The issue isn't evenly distributed. It's not like cars in traffic or whatever.

Airlines put the expensive seats in the front. The people who can afford them are usually much older, either traveling retirees or very late career white collar workers who have significant status. They're the first ones holding up everyone because they take forever to find all the assorted shit (personal item, oversized roller bag, neck pillow, laptop, ipad, lost earbud, etc) they've stuck all over the place, which the gate agent/FAs wouldn't admonish them for because of their aforementioned status. But they're first class, so the peasants behind them can wait in the bread line.

After they get off (on watching you glare), depending on airline, it's the fraction of people who are old and not rich, or don't fly often and aren't used to all the ritual. They'll have placed their bag in an overhead that's 12 rows behind them and demand everyone stop and crowd surf it up or else they'll just sit there blocking the line.

After them come the young vacation families, you know, the ones who had the screaming baby for the last 6 hours. They couldn't be bothered to pay for seat selection to save money so one parent is with one kid three rows ahead but needs to coral the kids behind them because the other parent was playing on a Nintendo switch for the whole flight and didn't try to organize all the kids toys, now lost to entropy, and so the marital spat and bawling (louder now) children begin.

Then there's you. You fly a lot so you have nothing more than two pairs of underwear and a toothbrush, all safely hidden from the TSA in your prison wallet and ready to go without so much as a nanosecond of notice, along with your phone and airpods to combat the screaming child in front of you. You got 31B, way in the back, after trying to game united's seat assignment system by checking in only after all but the exit row seats were taken, but someone missed their flight and here you are.

Generally the legacy airlines will have the most old people, but the vast majority of people on them are very used to flying, because they know better than to book a budget airline. It'll be slow yet ordered.

The budget airlines like united and frontier will be the opposite, lots of young spry 20 somethings, but lots of vacation families that couldn't afford Delta... I won't sugar coat it, it's gonna be a shit storm. The FAs have been contractually required to keep everyone at the very edge of their sanity through the enforcement of a variety of draconian company policies (like turning on all the lights half way through a redeye to scream about some credit card offer), so things are primed for chaos. Lots of shoving and yelling. Everyone's reviewing the Wikipedia "list of crimes of passion" to see if this qualifies.

Then there's spirit. Half the people on the flight will be coming down off of something they got on the dark web by the time you arrive at the gate. You've already seen at least a liter of blood spilled from various fist fights. Everyone was already up and crushing each other in the aisle long before the captain even briefed the approach. The FAs have locked themselves in the lavs by now and the captain (an FFDO) has barricaded the flight deck with charts and duct tape and is aiming his questionably modded P320 at he door. Welcome to the new season of Hunger Games - Spam Can. You're on your own, good luck and good hunting.