I'll just mention, that the true Threepersons Style holster can't have been any faster than the earlier ones, as long as the grip is above the belt:
rt frazier (1).jpg An R.T. Frazier.
These were created to provide more protection for the pistol itself, so covering the hoop of the trigger guard would have seemed the obvious thing to do.
As long as the grip is fully exposed, and the mouth of the holster no higher than the rear sight, and there is no strap that must be unsnapped (sometimes known as a 'retreat strap'), what more would be needed?
In the end the style was made famous as a Threepersons by Sam Myres; but it was created for the Texas Rangers as a concealment holster on a narrow trousers belt; and popularised by the FBI who wore it the same way. Much has been made by other historians about the Threepersons being created for the automobile; seemed logical in retrospect but the evidence it was created for another well-known reason: by then it was no longer p.c. to carry a pistol and ammo exposed (which is ironic given today's milieu).
As we see from Tom's pic, gunmen of the time placed the belt to position the grip just where their gun hand would fall to it.
Little known fact (now I DO sound like Cliff Claven vs. Sheldon Cooper): the Threepersons, best illustrated by the Jordan, provides huge leverage against the stitching in the welt, when lawmen leaned on the pistol grip; as they did. Jordans were also known for breaking ribs when the lawman jumped down into something and caught the holster muzzle: the full-length metal shank that the original Border Holster (Askins) didn't have!