The load/unload time shouldn't be affected too much, if the person can't fit in the belt, they probably can't fit the bar either, the people that won't realize the seatbelt is there will probably be the same people that leave their bar up until the attendant gets there and there is at least one person on every train that doesn't understand "push down and pull up" on every train so the unload time shouldn't be affected with the extra step of unbuckling the seatbelt.
I'm one of those people who leave my bar up until the person comes.