Just to pick up on the physics in this thread:
For any particular speed e.g. 40 km/h, the power you have to put in to the bike (the energy per second) is the same whatever cranks you have on and whatever gear you are in (ignoring tiny things like chainline efficiency).
The cranks also have no effect on the RPM needed for a particular speed in a particular gear. If you are in 54 front and 15 rear and doing 90 RPM then you’ll be doing around 40 km/h on any cranks. Try Sheldon Brown’s gear calculator if you don’t believe me. Doka, if you’ve reduced your rpm then either you have slowed down, or you are choosing bigger gears to compensate - but that’s perfectly possible.
Crank length does affect how hard you have to push. With a longer crank you need to apply less force to hold the same speed in the same gear, compared to a shorter crank. However, you have to move that force through a longer distance. It’s not intuitive but the two things cancel out in terms of the physics so moving the smaller force through the longer distance requires the same power as having shorter cranks and moving a larger force through a shorter distance.
So there is no gain from different cranks in terms of the physics. The biomechanics is a whole different story but it becomes an empirical question, I think.