Yes, cardio and metabolic exercise are important in burning fat, but there are certain thresholds in a heart beat which everyone has. This is called the fat burning zone and is actually stimulated more with long slow distance cardio. When you do not control your breathing you decrease the amount of oxygen to the muscles who’ve in turn causes a faster build up of lactic acid, resulting in quicker fatigue. The mouth should be thought of as a vent (exhale only). Although it is important to increase your heart rate, it is a machine and can only work so fast until other structures are deprived.