You are intuitively correct.
No, they do not. As long as you can inhale, you are getting oxygen. The mask does not filter oxygen out. It is not a selectively permeable membrane. It does not keep people from taking a full, deep breath. It does not change your tidal volume. It does not change FiO2.
People might have to breath at a slower rate, bc it takes more time for air to pass through a mask. That effects exercise, not general everyday activity like walking. It clearly doesn’t stop people from talking (it is not a ball gag).
Last night I wore my mask for 12 hours. A FITTED N95. I ran down the hall. I helped move obese patients. I talked on the telephone. Luckily, I did not do CPR last night, but I have done CPR in my N95. I have worn an oxygen saturation probe, and my sat never changed. I have worn an ETCO2 monitor, and my ETCO2 did not change. I have worn a face shield as well as my N95 for over an hour at a time, in a plastic gown, in a closed patient room, while helping intubate patients and taking care of their nursing needs, while starting IVs, swabbing for covid19, with no breaks and no hydration. I did not die. I did not pass out. I did not get dizzy. I did not get hypercarbic, nor hypoxemic.
Yes, I am fit. Yes, I am not obese. Yes, I can run a marathon. Yes, wearing that N95 while doing CPR sucks, while running upstairs to the OR sucks, while pushing stretchers with 300# people on them sucks. So you go a little slower, you move a little more carefully, and you breathe a few more times per minute.
N95s, and masks in general, barely increase dead space. They are air permeable, and CO2 is a LOT smaller than a virus. CO2 passes through a mask, or everyone wearing a fitted mask would die.
Masks make people feel claustrophobic (me, too). Masks make our inhaled air warmer, and that also changes how we feel about breathing. Masks that are harder to pull air through (like an N95 compared to a surgical mask) mean you have to increase your inhalation period, and more forcefully exhale, which can change mask feel and definately change our perception of breathing. Masks are imperfect, even N95s. Even P100s. But the argument that someone can’t wear a home-made or surgical mask, at the grocery store, bc they have a “medical condtion” is raging nonsense. The argument that it affects your thinking goes against the reality of OR staff wearing masks throughout mutli-hour operations.
Here’s an overly specific, overly technical discussion of alveolar dead space, functional dead space, gas perfusion throughout these spaces during respiration, etc: https://journals.lww.com/anesthesia-analgesia/fulltext/2012/04000/rationale_of_dead_space_measurement_by_volumetric.29.aspx