I travel about once a month to the US and back to Canada... the security lineup can take up to an hour sometimes leaving SFO. So if I don't want to have a chance to miss my flight, I have to arrive 2 hours early really.
ATL on the other hand....even with TSA pre I can take a full hour.
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.
I don't even want to think about trying to get through at a reasonable hour.