Reading / viewing up on how jet engines work, this video explains at the 9:02 mark that, for turbofan engines, ".. it is more aerodynamically efficient to have a lot of air moving relatively slowly rather than a little air moving relatively quickly." I.e. a lot of air bypasses the combustion chamber and is only accelerated by the main frontal fan.
This claim has me puzzled, given that kinetic energy is dependent on the square of the velocity, i.e. $E_k = \frac12 m v^2$, I would have guessed the inverse. Does "aerodynamically efficient" mean something very specific here?