I'd have to question that. Drivers do not just go wildly from place to place or be assigned runs individually unless there is an emergency of some sort. They have a schedule that tells them in advance where they are supposed to be their entire shift. That includes breaks which are loosely scheduled by time, but, are subject to when the previous to the break run is completed. Individual trip assignment would require a room full of dispatchers and plans that would make D-Day seem like a walk in the park.
You make it sound so random. It is not random at all. I can't speak for Disney, but, I know that most work on a seniority basis, with highest seniority getting to chose their run setup first, then so on after that. What was described by the poster as a problem with the driver more then likely was a newbie. Just starting out and unsure of the directions and turns that everyone gets familiar with over time and desperately not wanting to upset anyone by accidentally shooting by a turn and having to double back.
So in short, I cannot accept that going faster will prevent them from having a break. Just the opposite is more likely they outcome of driving to slow.
If they worked on a published times schedule, they may be moving slow to not get to a location to early. (and in a way they are, you just don't know what that time goal is) Ever been on an airplane the was sitting on the pavement for extended times because there were no gates available. Kinda the same thing, getting there early can be a hassle as well if the previous buses haven't cleared out. But, I don't think that was what this was. Having driven around in WDW I can tell you that there are some massively crazy drivers running around there. An accident could delay your trip by a whole lot.