The obvious is heartworms. Although in severe cases of intestinal infestations pressure can be exerted on the diaphram causing rapid breathing. Worms can also migrate to the lungs causing problems there.

The other health problem would be a low grade lung infection. I went through this after my previous dog was diagnosed with Lymphoma. The blood work showed no signs of infection after she went into remission. But yet she was panting hard. Just happened by chance she got a paw infection, and Clavamox not only cleared that up, but also her breathing problem.

My current Clover dog (1/2 GP, Lab, plus something) panted like wild when she was < 6 months of age. Eventually she out grew it. So yes it can be breed related.