Respiratory gasses interact with the respiratory system through their partial pressures.
Room air is 21% O2 @ 760mmHg, or a partial pressure of 160mmHg. So, at first glance, it would seem that breathing 100% O2 at 160mmHg (3.15psi) would be physiologically identical. However, the lungs are at body temperature and wet inside so they are filled with water vapor at the partial pressure of water for that temperature (60mmHg). If you breathe room air at 760mmHg (ppO2 = 160), it gets diluted with water vapor so you end up with ppO2 of 100mmHg in your lungs.
This water vapor effect is what puts a limit on breathing pure O2 in non-pressurized aircraft.
If you want your space suit to provide the same arterial ppO2 as atmospheric room air, you need slightly higher pressure… about 220mmHg (4.25psi).
There is the added issue of “dead space” and re-breathing exhaled CO2, so fudge that number up a bit more. 5.0psi is a round nice number, and happens to be what the 100%O2 atmosphere was in Apollo capsules.
Your respiratory rate is driven by blood ppCO2, not ppO2. If your breathing gas supply was switched to 100% N2, you would have only a slight sensation of something wrong (like you needed to yawn) just before you passed out. (see inert gas asphyxiation) Then you would die. So, as long as the suit’s CO2 scrubber is working, I think the astronaut would be oblivious to low level of ppO2. I suspect a slow suit leak down to vacuum would go unnoticed from a respiratory perspective. But the suit would become delightfully mobile with the lower pressure.