French company Seabike has developed a swimming device that uses your own leg power to accelerate you through the water at superhuman speeds. This crank-driven pusher prop looks a bit like an underwater unicycle... We'd love to take one for a spin!
I don't really have a good gauge of how fast you can typically go with swim slippers though. Comparing to a person swimming without any aides would be much more useful for getting an idea of what "superhuman speed" means.
Some videos in the article show a guy cruising while barely using his arms, and then SCUBA divers only pedaling with their feet but they were pedaling slower