The poster on the WSJ comment board said that there is a rapid response team stationed in Southern Italy. He said that it would take ten minutes to activate the team and 20 minutes flight time. They had pictures from the drone and knew exactly where and who every one was in real time. There was no problem identifying anyone. The attack was on-going for 6 hours. There was time to get some kind of defence going. They were watching the attack both from the drone and from cameras inside the compound. They knew everything there was to know in real time.
There was nothing in Tripoli. They had taken everything away. The ambassador had requested that they at least return one plane and they refused.
That is a speed of 1350 knots, almost twice the speed of sound. Most transport aircraft cruise at about 300-480 kts at altitude (a bit more in an emergency situation) depending on aircraft model. Helicopters cruise at considerably less. Once you get there, you have to find a suitable place to land and deploy - unless you are going to jump in, and then you have to have a suitable place to drop in to and a plan. Likely, the best plan in Benghazi would have been to land at the airport and deploy to the counsulate/annex in force from there.
There is little doubt, however, if the troops and transport were on alert and prepped they could have been in the area and in the fight in time to make a difference. Obviously, they weren't - so somebody, somewhere made the decision not to employ. The next questions are, "who" and "why"?
It's not like it was a secret at the time what was happening. All the applicable Command Centers in the Med and around the world would have been buzzing and especially as there was a drone on station watching live time. This would have been a top priority.