Fast breathing while moving a distance.

My husband tested positive and for a week he didn't took care of himself & after consulting with Dr we got to know his 50% lungs have been infected and started his medication as per Dr advice. Today is 5th day of the course & 12th day of being infected with Covid-19 & he's doing real well but his breathing get fast after moving a little distance (saans phulna), he starts to sweat & I always see him keeping his hand on chest in this condition. He gets calm after 2 minutes. What could be the reason for this? He used to be a smoker (5-6 cigarette a day) before Covid-19. His saturation is good (stays above 94 throughout the day). Mucus is coming out due to medicines. He's taking steam & nebuliser as per Dr advice.