I don't worry too much about flushing my black tank, like I said I usually only flush it once if there is a line. But, last time I checked I paid to use the campground and the dump station so if I take 10 minutes longer to flush my tank until I'm satisfied so be it. If someone can't wait or doesn't like it there are dump stations at flying j's or other rv parks.
Some worry about people taking too long at the dump station, I get tired of listening to people tell stories at the check in station. I've waited longer at the check in numerous times than I have at dump stations.