1. Go green.
2. Have you seen how fast someone can cover the distance in a self defense situation?... we are not talking about the OP engaging a threat 50 yards away.
To that, you have to add the added time to move the shirt out of the way and draw, this takes time. Add having to rack the slide, and you are probably dead.
A USPSA competitor shooter, someone that is above average but not the best and has a nice competition rig with no obstructions takes just under 1 second from draw to shoot. Keep in mind, this is using a competition rig that barely holds the gun, no shirt removal, no twisting of the body (if you are carrying at the 4 or 5 o'clock).
Let's assume in a defense situation carrying concealed it takes double that time, lets call it 2 seconds. Racking the slide probably adds at least half a second. (And hope the slide doesn't slip and you have to rack it again).
Go outside and see how much distance you can cover in 2.5 seconds.
2.5 seconds is probably very generous as most people barely practice.