One of my closest friends just flew out of Midway last week to Vegas, and I believe she was on ATA. She said the security was very tight, she was frisked (extensively) at two checkpoints. Maybe because she was flying alone..?? She mentioned that the flight itself was great, and both coming and going, the plane actually left early "ladies and gentlemen, it appears that everyone is on board and in their seats, and with that we will leave early", so she arrived both times 20 minutes earlier than scheduled.
She did also mention however that the airport in Vegas has way more security, which I think is normal for the larger airports, like in Orlando, Chicago and NY, amongst others. I would still recommend that you get there several hours early.
Have a great trip.
On a side note, she completly forgot a nail file in her purse, and was almost denied boarding privledges after being interrogated in a side room with agents. So ladies PLEASE remember to go thru your purse and carry-ons to get rid of any nail files, tweezers, etc.!!!