My father always used to say that an asthma sufferer will either have the worst forced vital capacity in the group, or the best.
He taught fitness techniques to athletes so he had a lot of experience with this kind of thing. And he found that many of the breathing exercises that asthma sufferers do to make their lives easier, can be useful for athletes or ordinary people.
If someone with asthma does them regularly, they might even end up with a better capacity than most people. It's good to know even that kind of disadvantage can be overcome.