Technically the way to think about latency is that a process has N serial steps and you can (a) reduce N, (b) run some of those serial steps in parallel, and (c) speed up the steps.
For one thing, different aspects of the organization own the N steps. You might have one step that is difficult to improve because of organizational issues and then the excuses come in... Step 3 takes 2.0 sec, so why bother reducing Step 5 from 0.5 sec to 0.1 sec? On top of that we valorize "slow food" [1] have sayings like "all good things come to those who wait" and tend to think people are morally superior for waiting as opposed to "get you ass out of the line so we can serve other customers quickly" (e.g. truly empathetic, compassionate, etc.)
Maybe the ultimate expression of the American bad attitude is how you have to wait 20 minutes to board a plane because they have a complicated procedure with 9 priority levels and they have to pay somebody to explain that if you are a veteran you are in zone 3 and if you have this credit card from an another airline that this airline acquired you are in zone 5, etc... meanwhile they are paying the flight crew to wait, paying the ground crew to wait, etc. Southwest Airlines used to have a reasonable and optimized boarding scheme but they gave up on it, I guess the revenue from those credit cards is worth too much.
[1] it's a running gag when I go to a McDonalds in a distant city that it takes forever compared to, I dunno, Sweetgreens, even "fast" food isn't fast anymore. When I worked at a BK circa 1988, we cooked burgers ahead of time and stored them in a steam tray for up to ten minutes and then put condiments on them and put them in a box on a heat chute for up to another ten minutes. Whether you ordered a standard or customized burger you'd usually get it quickly, whereas burger restaurants today all cook the beef to order which just plain takes a while, longer than it takes to assemble a burrito at Chipotle.
Japan has its own versions of these things so I doubt it's this. The whole culture is, in general, not built for efficiency either.
The purpose of the many boarding groups is IMHO, to make those in groups > 1 feel as though they're missing out on some perk that they could get if they paid more. It's an intentional class system where some are encouraged to look down on those who paid less, and vice versa. It's good for revenue, bad for people.
Airline boarding is not the only class system in play. At every level of government, even within transit agencies, transit and its customers are seen as and treated as second class citizens. The idea of investing money, time or energy to shave even scores of minutes off the commute of someone who uses a bus, often seems as if it’s an unthinkable thought in these organizations.
Most flight attendant and pilot union contracts only pay them based on the hours with the door closed or in flight. (This is changing, but it's how it's been for a long time.) This reduces the incentives for quick boarding, as most of the flight crew is not being paid for that time.
Unlikely, most cities transport systems will run into issues with capacity long before they run into issues with ticket gate latency. No point getting people through the gates faster if they’re just gonna pile up on the platform and cause a crush hazard.
At peak hours in London, the inbound gates are often closed periodically to prevent crowding issues in major stations. If you look at normal TfL stations you’ll notice there’s normally a 2:1 ratio of infrastructure for people leaving a station vs entering. Because crowding is by far the biggest most dangerous risk in a major metro system, and also the biggest bottleneck.
Here's an example video to show the gates in action: https://www.youtube.com/watch?v=YffjxN3KsD4