I've spent way more time transiting ATL vs. actually being an O&D passenger at ATL but I'm always amazed at how much a cluster ATL is in those instances.
Last time we were there for a wedding and went to get the rental car and then it almost took us an hour to get back up to the terminal to pick up my brother-in-law and sister-in-law who landed after us (we would've been better off just waiting for them and then going all together to get the rental car).
Similarly on departure that trip I recall the PreCheck line being insanely long.
I'm spoiled being RDU based where I can usually roll up to the airport ~10 minutes before boarding time and get through PreCheck with time to spare to go to the SkyClub for a quick coffee or drink