Also, 15 min rounds may simply be a provider's way of being more budget friendly. It doesn't NECESSARILY mean she's high volume.
Seeing say, 4 guys over 2 days for 15 min at 100 per, for example, is still $400 in the pocket. Better than nothing. Sometimes pennies are pinched so tight that any remotely affordable rate would look good when half hour or less sessions are available. A guy may want some action, but JUST CAN'T part with 2 (or more) bills for an hour session......but ah, 1 bill...15 or 30 min....yeah, I can manage that.
Some (money made) is better than none.