Let's try an (thought) experiment:
Put a tennis racket in front of your face. Does it impede airflow? I think the answer is clearly no.
Put a plastic bag over the your nose and mouth? Does it impede airflow? I think the answer is clear yes.
Put a food sieve over your mouth and nose. Does it impede airflow? I think the answer is probably no, but there's room for doubt.
Clearly, something placed over your mouth and nose does not necessarily impede airflow - it depends on the properties of the material it is made of. Some materials obviously have no impact, some have a lot, others are inbetween.
So the question is: why you conclude without careful measurement that a given mask would impede airflow? Presumably you have some intuition about the material and what you know about its properties ("It stops X percent of all particles above a certain size!") that lead you to conclude that it's somewhere between a food sieve and plastic bag, rather than a tennis racket and a food sieve. But there's no "common sense" here. How can Goretex fabric allow water vapor to pass freely, but not allow water in liquid phase to pass at all? You cannot explain that using common sense, but again, you have some intuition that you can use common sense to reason about the properties of masks.
I should also stress that the 20-30% number comes from a 2009 paper on the reduction in airflow. The increase in breathing effort could be less than that, unchanged or more than that, depending on a large number of factors. Example: sitting on a sofa in a comfortable temperature and being very relaxed ... you are already breathing fairly shallowly, and the increase required to overcome whatever effect the mask has will move you only to a condition you are in very regularly anyway, and so will likely be unnoticeable (there's also the "X percent of a small number is a small number" aspect). By contrast, if you are exercising near V02max levels and in excellent cardiovascular condition, you are likely already breathing almost as hard as you can, and so breathing harder due to reduced airflow is likely to challenging to impossible.