My default assumption would be that if the mask doesn't make it more difficult to breathe then it is unlikely to be effective. The filtration mechanism should introduce friction which you then have to compensate for with additional pressure in your lungs.
There is absolutely no reason to believe that effectiveness at preventing infection is linearly correlated with difficulty breathing or that to a degree that the additional friction induces difficulty must rise to the level of preventing breath in order to be effective. You aren't preventing 50% of particulates from getting in by preventing 50% of oxygen for example.
For example I imagine that you didn't believe that doctors wearing masks pre pandemic or construction workers weren't just going without breathing well for hours given paucity of air becomes an problem very very quickly
"Expect some discomfort - that lets you know the mask is working"
As a statement of technically accurate fact: I wouldn’t have been able to imagine how easy it is to breathe through a 3M Aura 9320+ FFP2 mask. Not before trying one. It’s… magic? –Feels like magic! These ones: https://www.3m.com/3M/en_LB/p/d/v000078838/ – pretty much the same mask as the 9205+ N95: https://www.3m.com/3M/en_US/p/d/v101146024/
“Duckbill” type masks are great too, like Kimberly-Clark / Kimtech pouch respirators: https://www.kcprofessional.com/en-us/products/scientific-and... … These might be the most comfortable article of clothing I’ve ever worn? Like really seriously. Very easy to breathe through. No drag or inertia at all. The 3M Aura mask has the edge in breathability though – feels impossibly transparent. I believe it’s due to the material itself. The duckbill type gets its breathability through using more material and a bigger airspace. I’d love to try a duckbill made out of the material in the 3M Aura. (Secret tip: If the duckbill is creased just right it looks more like a Stormtrooper than a duck. The free bread is nice though.)
I’m reading the Wikipedia page on HEPA filters now. Trying to get a better feel for how the material works. Seems like part of how the close-fitting masks can be made so easy to breathe through is that the possible speed of air flowing through is limited. Anywhere in the range of airspeed that we can breathe aerosolized particles in or out, the filter material causes slight turbulence in the atmospheric gas, it flows and eddies around the filter material – the air flows around the material – but the aerosol particles have much more inertia and hit the filter. I actually hadn’t understood or appreciated this aspect before now.