I saw some monofin competition results so you can do the math
50m sprint - 16 sec
6 km - 1 hour
And plus one my experience.
Once I was doing my laps swimming in the pool and in the other line some swimmer did her training. Well, I was swimming crawlstroke and she did dolphin style without arms (those stretched forward) without fins and on her back. I hate to admit it but she was faster then me.
