I've thought about this a bit. I think it's slow loading for a few reasons.
The cars are spaced relatively far apart. I believe this is mostly because they are very open (unlike the large shell that's on Haunted Mansion and Mermaid), and not feel crowded or have other people in your line of vision. I realize Buzz has cars much closer together, but people are so busy playing the game, I think it's less of an issue.
Since they only specifically only fit one or two people, a lot of large groups and families are suddenly faced with the decision of who gets to ride with who. I've seen many empty cars pass by as groups discussed this right at the load. You'd think the fact that they can plainly see the cars only seat two before they get to load would make them think about that a bit more, but no.
The queue is super engaging... maybe a bit too engaging. People are so caught up interacting with the queue, it also slows down the line.
and...as contrasted with the original trackless idea concept that Universal estimated at max 1,980 capacity per hour. ...But, thematically Universal did do a nice job with the omnimover version.
...except for the capacity issue.
My guess has been that lack of strong merchandise sales at Orlando, which they pushed hard after the first movie, had a lot to do with it. Even the first trackless concept drawings/computer generated run
were originally just listed for Hollywood (where Nintendo is now), and not Orlando....and no Yoshii in Hollywood may have a bit to do with the present SLOP, since it's a somewhat similar system.