People with obstructive lung disease have shortness of breath due to difficulty exhaling all the air from the lungs. Because of damages to the lungs or narrowing of the airways inside the lungs, exhaled air comes out more slowly than normal. At the end of a full exhalation, an abnormally high amount of air may still linger in the lungs.
Obstructive lung disease makes it harder to breathe, especially during increased activity or exertion. As the rate of breathing increases, there is less time to breathe all the air out before the next inhalation.
Obstructive and Restrictive Lung Disease, WebMD, https://www.webmd.com/lung/obstructive-and-restrictive-lung-disease#1