j-valve
Contributor
Most cars are most efficient in 5th gear(overdrive) at around 90km/h(45ish mph).Just like a car, the faster you go the less efficient you are, regardless of how fit you are.
I haven't seen you swim saying, but the overall speed diference may be more than exertion or effort. Gear config, body streamlining, PROPER kicks, and body angle are all things that can lead to major difference in overall difference of speed between 2 divers.
Just my 2c
J