Why this formula says jet engine is efficient at any speed?
This is the result from the simple momentum balance. In order for the propulsor to produce thrust, the exit speed after the propulsing element ($v_e$) must be higher than the incoming speed ($v$):

$$T=\dot{m}(v_e-v)$$

where $\dot{m}$ is the total mass flux through the propulsor.

So the correct way to read the efficiency formula is: the closer the exit speed is to the incoming speed, the more efficient it is! Therefore, the more mass flux you can generate at a smaller speed difference, the more efficiency you can attain without sacrificing the thrust. This is the overarching reason why bypass ratio makes engines more efficient.