My Elsie is in fabulous shape at 11.5. Gets walked 2 times a day - 1 mile each walk. (we do not walk in the heat of the day. In H H H weather, we do walks at 7 AM and 11PM. She is a panter. She pants if we do not walk. She pants if walk. Sometimes her panting is so bad, I have to turn a fan on to drown out the sounds of her panting or I can't sleep!
I had her checked out when she was newly adopted and of course she has been seen several times since then. She is just a panter!
Greyhounds are sprinters, not long distance marathon runners. This is very typical of the breed. 5 minutes of play, 6 hours of sleeping it off!
Sounds normal to me, just please be very careful about the heat. They are not capable of handling the heat or cold the way your other pup is.