[Ekaros]

Isn't same true for any mode of travel? At least terrestrial. Optimal speed is not either top or slowest speed.

Lot of factors affect efficiency, but almost certainly neither very slow or maximum speed is most efficient.

[fer]

It's true for most machines (as in, energy conversion).

The main exception would be heating, and even then, it's usually that the optimum performance is just really close to the top performance; e.g. imperfect combustion of more fuel being less efficient than perfect combustion of less.

[chrisco255]

What determines "slow" or "fast"? These are subjective quantities.