SFO is the only airport manned by private security instead of TSA, that is the reason it is super efficient.
I've cleared from the entrance through security to my (domestic) gate at SFO twice in ~15 minutes. It really blows my mind how they pull that off.
Better training, more incentives to be passenger focused etc. might be the reasons.
In fact in some contests these private guards performed 5x better than TSA employees in catching the bad guys/bad items while actually doing it lot faster.
For ATL - you can go in through international terminal and take the tram to the domestic gates. There is just less parking on the international side.
Wow, I just flew out of IAD and even though I arrived at 4AM for my 5:15 flight, security still took 45 minutes. The line was already twice as long by the time I got through.
I don't even want to think about trying to get through at a reasonable hour.
ATL on the other hand....even with TSA pre I can take a full hour.