User blog comment:Marcus Junior/Mermaid at whitebeard level?/@comment-1595565-20111020192739

The tail makes them more flexible in water than legs do for humans, so they would be able to use more muscles and therefore go faster. I think only mermaids are fast because most of the time women are naturally smaller and lighter than men. As for the top speed, there's no way to tell since no mermaid's speed has ever been clocked.