I'm not really sure why so many people in parts of the US had to stand in line for hours. I'm guessing that, at least with the early voters, who voted on the weekends leading up to the election, they were simply unprepared for the number of people who decided to take advantage of the early voting. They just didn't have enough voting booths or people checking voters in, or something. Also, you used to have to register to vote well in advance of the election, but now you can register at the polls. This takes a few minutes per person.
Here's what happened when I voted this week, for the first time since I moved to New Hampshire. The voting was at an assisted living facility. Other polls in my city were at schools or the Elks Club. The polls opened right when I had to be at work, so I had to go after work. Got there about 4:15, there was a line out the door. I stood in line for about 20 minutes. Then I had to stand in a shorter line, based on the first initial of my last name, in order to be checked off the voting list. Got my paper ballot, went to a voting booth (with a curtain) and voted. Put the ballot into the ballot box and walked out. Took about 40 minutes.
Had I been able to get there during the day, I suspect it would have taken a lot less time. I'm also in the largest ward in the city, so there are simply more people to process. Many people work at a distance from where they vote, so the evening hours at the polls can get very busy.
There are variations on this. In one state, we had voting machines inside curtained booths. In another, the booths didn't have curtains. Some states have made me show a picture ID, some haven't.
This year, at this polling place, after you voted you could browse a book sale and get a flu shot.