How hard to breathe with a collapsed lung.?

Depends: Difficutly in breathing can vary significantly depending on: degree of collapse or how much functional lung is compromised. How functional the remaining lung is.(is there already underlying lung disease?) cause of collapse and associated chest wall problems. Air, fluid or blood around the lung or an airway blockage. So, it can be symptom free or life threatening.

Little to a lot: "collapsed lungs" (pneumothorax) come in varying sizes and degrees of seriousness. Smaller ones may be only slightly noticeable, with a little air hunger an perhaps cheat pain. Large ones can be life threatening!