Speed is only one advantage. Once this mounting technique was well practiced, we went into an indoor range. A single light bulb in the next room cast a faint glow on the range so you could just tell where the targets were. You could not see the sights. Yet, from the ready position, everyone in class put every shot on center mass.
Police officers practice reloading their pistols quickly.
If you have fired several shots and are covering a suspect, your gun is in the ready position. Open the action. That is the loading position. Knowing you have a shell in the chamber, you can get shells from your pocket and stuff them one at a time into the loading port with your weak hand. You can feel the rim end of the shell to know it is pointed in the right direction.