Crowding out O2 isn't going to be meaningful, the amount of CO2 is simply too small. If there's an effect it's from reducing the rate at which the body can expel the CO2.
If crowding out O2 was relevant why do I feel the same on our local mountain at 10,000'+ vs 0'- in Death Valley? To crowd out that much O2 with CO2 would be lethal. (That's not to say that my performance is the same. There's a big difference in the heart rate I can sustain.) What we feel is the CO2 level in our blood rising because it isn't diffusing into the lungs. Lowering O2 is only detectable with training and that's based on noting the symptoms of oxygen deficiency on brain function. (Such training is relevant in the world of aviation where it might give warning that you need to grab that oxygen mask. The average person will never encounter such conditions, nor have the resources at hand to make use of the knowledge even if they did realize it.)
If crowding out O2 was relevant why do I feel the same on our local mountain at 10,000'+ vs 0'- in Death Valley? To crowd out that much O2 with CO2 would be lethal. (That's not to say that my performance is the same. There's a big difference in the heart rate I can sustain.) What we feel is the CO2 level in our blood rising because it isn't diffusing into the lungs. Lowering O2 is only detectable with training and that's based on noting the symptoms of oxygen deficiency on brain function. (Such training is relevant in the world of aviation where it might give warning that you need to grab that oxygen mask. The average person will never encounter such conditions, nor have the resources at hand to make use of the knowledge even if they did realize it.)