"Yet a mask makes it harder to inhale the quantity of air needed to perform at the highest levels. We know that wearing a surgical mask can increase the resistance to airflow. Exercise invariably leads to faster and harder breaths, so wearing a mask during exercise places a further strain on airflow.
At low to moderate-intensity exercise, effort will feel slightly harder than normal with a mask, but you can still walk comfortably. The challenge appears to be more during heavy exercise (say, rugby or football) taking in air at rates of about 40-100 litres per minute."