A bolt action rifle might have slowed down Lanza, but it wouldn't have stopped him. Enter the school, shoot anyone in his path. Kill the teacher, block the doorway, he now has a room full of weak targets that he could kill at whichever speed was comfortable to him.
As for the theater, the killer's AR-15 jammed right off the bat. He did all his shooting with a pump action shotgun and a handgun. Just as much opportunity for someone to stop him then as if he had a bolt action rifle.
People don't grab for the gun, or run away when a killer is reloading. They go full on deer in the headlights, duck down and stay in place. Maybe behind cover. But they won't move when the shooter approaches them.