There is already a bottleneck on these trains because the staff need to bring you to the compartments. Unlike regular trains there is a lot of staff (one per carriage).
Boarding in Zürich you can only enter in one door for the sleeper.
Also most stations are exit only and no new passengers can board.
> There is already a bottleneck on these trains because the staff need to bring you to the compartments.
No, that's just not how it works most of the time with ÖBB, not even in Nightjet. I walk in and the ticket inspection happens later. Yes there's staff outside with NJ to sort you in, but you can walk around them. At no point are they a real bottleneck.
Boarding in Zürich you can only enter in one door for the sleeper.
Also most stations are exit only and no new passengers can board.