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.
If there were ships in the Mediterranean Sea looks like Benghazi is is right on the coast. Looks like they may have been near enough to help out. But I don’t know what planes are available on what ships.
From the testimony before Congress by members of the State Dept:
Some officials say the U.S. could have sent military forces to Benghazi from U.S. Naval Air Station Sigonella, which is some 450 miles away in Sicily, or mobilized a Marine team in Rota, Spain. Some officials said the U.S. could also have sent aircraft to the scene in a “show of force” to scare off the attackers. State Department officials dismissed the suggestions as unrealistic. “They would not have gotten there in two hours, four hours or six hours. They don’t have troops sitting in a room next to an airplane with a pilot just sitting in the next room drinking coffee,” one senior State Department official said.”
The problem is that other people say that there is a rapid response team stationed in Southern Italy just for such emergencies.