Using Mr. Preston’s own time line (below), pretty much any decent firearms school in America can teach you how to stop a lethal threat in well under 15 seconds or less (most can teach you how to do it well under 5.)
Assuming the shooter did shoot 20 rounds, he probably had a reload or two to deal with (don’t know the make of the 45, so I will assume around 7 rounds in a magazine), which is still plenty of time for someone to draw from the concealed and take care of the problem.
"We know very little yet about what set the tragic events in motion. But reports have consistently stated that the killer fired about 20 rounds from a privately-bought .45 caliber semiautomatic Smith & Wesson pistol.
It takes almost no time at all to put 20 rounds through a semi-automatic pistol. Even someone who is not all that competent with a gun can put 20 rounds through a semiauto firearm in around 15 seconds or less."
(Via.) The PJ Tatler