Here’s a look at the BUD/S obstacle course as run through by an actual SEAL wearing a helmet cam.
In training, a class will line up by their last recorded time and adjust in rank according to how fast or slow they are. Imagine 200 guys staggered; it gets jammed up really quickly, with guys clawing over each other for a faster time. The good news is that in BUD/S, the class size gets small really fast, too, so the obstacle course becomes less crowded.
We would typically bury the slowest guy (by instruction) at the finish line, leaving only his head showing, so he could watch everyone run over him. You did not want to be “that guy.”