fan - Air flow speed and penetration abilities

As I understand, two fans with the same diameter and with the same rotor speed(usually measured in RPM) can have different air pressure(usually measured in mmH2O) and air speed(usually measured in CFM) because of fan blades design. Higher air flow speed should provide worse penetration(for example, penetration to radiator with thin fin gaps) abilities. However, it is counterintuitive to imagine that air molecules with higher speed have smaller penetration ability than the ones with lower speed. Why is that so? Or is it because of complexity of air flow dynamics and air flow with slower speed has indeed for some (weird) reason better penetration abilities? At least I know that this is not because of Bernoulli's principle because this should apply only for tubes.

Answer

However, it is counterintuitive to imagine that air molecules with higher speed have smaller penetration ability than the ones with lower speed. Why is that so?

The velocity has an influence, but it is almost purely an issue of the shape of object which it needs to "penetrate". This influence is not even linear, but can even decrease when the velocity increases. It's all well educated in this old video; Fluid Dynamic of Drag, (Part I) At 8 min it's well shown.

Or is it because of complexity of air flow dynamics and air flow with slower speed has indeed for some (weird) reason better penetration abilities?

The decreased penetration ability comes mainly from so called "separation of the boudary layer", which shortly said simply causes a reduction for the open area of flow.

At least I know that this is not because of Bernoulli's principle because this should apply only for tubes.

Bernoulli apply everywhere.

ANSWER: Higher air speed $V$ always provides more "penetration" as this penetration is simply flow amount $Q$ through Area $A$, and $V/A=Q$. This "penetration"/flow might be really unefficient, the efficiency can be seen from the rise in the pressure. And basically the pressure generally rises along the velocity. But there is an optimum point. In water flow this is defined as a Froude number. At certain airspeed the area might be blocked because of separation, and if there is possibility to bypass flow, then the "penetration" is reduced.

It's not reasonable to answer more broadly to a question in this level; here's some material to study further;