If everybody steps onto the elevator at a set rate, momentarily stands still, then some of them begin walking forward iff there is space to do so, how does that

If everybody steps onto the elevator at a set rate, momentarily stands still, then some of them begin walking forward iff there is space to do so, how does that cause reduced throughput? To be clear, I'm not talking about some sort of "walk on the left" convention, which plainly reduces throughput. I'm talking about the "walk if you want to no matter what side you're on" convention, which seems to be far more universal.