Is breathing through a respirator/gas mask hard?

I frequently get questions and comments about how hard it is to breath through a respirator/gas mask. Unless you are a very young child or are using mask with clogged or poorly designed filters, breathing in a respirator is not difficult at all.
However if you are carrying out lots of physical exercise while wearing a respirator and a CBRN suit, you will overheat and become exhausted much faster than normal.