Air resistance increases quadratically to (according to the square of) the speed.
Thus, if you're taking shallow breaths (e.g., when sitting quietly in a chair), your mask may not significantly impede breathing.
If, however, you are exerting yourself (e.g., walking briskly), the resistance caused by the mask could easily increase 16-fold.
Regards,
If I’m on an airplane or a bus or whatever, I’m not likely to be walking briskly.