> enforcing boarding and paying in front and leaving via the back door, so the leavers don't delay the boarders.

In my experience, being able to pay at any of the doors increases throughput because people are not bunched up in a single door, neither to get on or off, Parallelizing (and load balancing!) the movement of people. Not having to tap to pay (either because it is free, because monthly passes don't require it or because you can pre-pay at a machine or online) would have some additional time gains at rush hour on popular lines.