get $100 Electronic shooting timer, Shooting Chrony
and you will discover that it takes you .20 second, at best, to react to the "beep" of the timer, if you are "wired", know what the signal will be, what to do when you percieve the signal, safety off, trigger half pulled and know that the signal will come in the next 5 seconds or less, and you are aimed in at the target, and the target is just an arm's length away. If you are at low ready, safety on (SA auto) or hammer down on DA, finger outside the guard, gun held in both hands at a 45 degree angle, so you can scan for multiple threats, it will take you .60 second or more to react to the signal, raise the gun to eye level and hit the chest vitals at 10 ft. A really, really fast man, who knows the trick of just flicking his wrists upward, without raising his arms at the shoulder, can get good hits in .45 second. The typical cop, tho, will need almost a full 1.0 second.
Now, add in the desire to NOT kill, not be part of a criminal action, not be civilly sued, bad light, having to DECIDE whom and when to shoot, etc, and it is likely to take 1.2-1.5 seconds to get a hit, even if you ARE gun in hand and scanning. That hit might not be a good hit, and your load is probably not all that effective, either. Another hit might take another .50 second or more. Any fit man can charge 20 ft in 2 seconds, and some can do so in 1.5 seconds. Some can take a step, and DIVE upon you from 10 ft in .70 second or less and a head on, diving target is VERY difficult to hit and it's VERY surprising to most people, too. Add in having to duck items thrown at you by 3 men, and you have no chance at all of hitting all 3 of them before one of them tackles you and makes you EAT that pistol. Granted, finding 3 such men in the same room, is extremely unlikely. In fact, having 3 such men in the same STATE aint very likely.