I'd just like to clarify that blood oxygen levels is not what makes you feel like you can't breathe. It's the amount of CO2 in the blood that's the issue. The first tweet addresses this point though.

I'm not entirely convinced by the test shown though. Doing a similar test while doing light exercise would be more convincing.

Another important factor to consider is that people breathe differently. Some people breathe more through their nose, others through their mouth. Maybe this can have an effect? Then there's also the fact that some people can take much stronger breaths (pulmonologists are always disappointed by my spirometry test results).

Mask wearing is important, but it doesn't feel right to just dismiss the reports that some people have more difficulty breathing with masks just because it doesn't happen to me.