There have been a few articles about airlines padding their flights times, and thus having increased "on-time performance". I don't personally fly much on Delta or Southwest, but on a recent award search I found Delta's flight times varied from 1h20 to 2h from SFO to LAX, with the 2h flight being at 8pm.
If you look at Southwest their flight ties are 1h20 to 1h30. There isn't much padding at all. AA is intermediate, going from 1h30 to 1h45.

I guess Delta is on-time now cause all its flights are longer (though not really longer).