Delta boards 35 minutes prior to departure. I do think the Atlanta airport is one of the easiest in the US to travel long distances between gates if you know your way around, and I cant recall a time it's ever taken me more than 25 minutes to get from walking off one plane to my next gate, but I still wouldn't book one with less than a 45-50 minute layover, because you never know which terminal each flight will use, and being a Delta hub, there are constant gate/terminal swaps last second. There are specific gates that may be 30-40 minutes apart if the airport is crowded and you arrive at the far end of the international terminal for example. Plus they close the plane doors 10-15 minutes prior to departure, so a 35 minute layover seems insufficient. If you end up with a 25 minute walk, plane will probably be sealed up before you get to it.
I almost booked one of these 35 minute layover flights just a couple days ago, and noped out when I noticed that during checkout. Charlotte is the one where I really want at least 75 minutes between flights because no tram, and the big and little planes tend to use opposite ends if the airport. But ATL, I still want 45-50 minutes. I had only 43 minutes in ATL on my most recent trip, and that was enough time to make it to my next gate in another terminal, grab a sandwich from the little kiosk nearest the gate, and get in line when my group was called, but I power walked it and did not do any dilly dallying whatsoever. And I didnt have time to wait for fast food takeout or anything.