I had to draw once but did not need to fire. I experienced a slowing of motion and tunnel vision.
I now believe the slow motion effect is caused by the brain speeding up. I felt as if I was moving very slow but my coworker said my draw was as fast as normal. At .5 second (my normal draw speed back then) I wouldn't have had time to process all the thoughts that went through my mind before realizing that the bad guy had a bb gun and ending my firing process.
Here's what I remember thinking (been almost ten years) I remember thinking how long it was taking to draw and raise my gun. I remember saying (in my head) front sight over and over. I remember thinking how skinny the man was and how concerned I was about missing (the guy was standing sideways). Finally I remember thinking "its only a bb gun" don't shoot. All that in .5 seconds. My only answer is that my brain must have speed up. If my brain sped up that might explain the slow motion effect that was only slow to me.