Bear in mind, this was with a medium stiffness (v. stiff by freediving standards!) monofin. The guy in question is quite a fast swimmer (ex-olympic development squad for swimming, so he likes going quick!) and was training in the pool for speedy constant weight ascents. A monofin displaces so much water that small amounts of efficiency loss can mean big problems. Another example of this is another swimmer that dropped half a stroke per length just from a monofin with five degrees more angle on the blade.

As for my timetrial with a mono, that's the plan for practice on Sunday