I've only really recently come to learn what mucus plugs are. In the last month, I've coughed up two "pea" sized round little balls of darker mucus. Before I coughed them out, my chest was very irritated and was coughing really bad for a few minutes until they got it. It's gross but I was so glad to have them out! I don't know if they affected my PFTs, probably not, but I felt winded but better after.