They can behave quite normal and perform at top level otherwise ~ all that's changed is their frame of reference.
The Pima shooter was stopped by apparently the Fish and Game folks that morning ~ for speeding ~ not for weaving back and forth. He may well have been out picking 'shrooms, or "weeds". Yes, Pima County Arizona is the world capital of natural drugs ~ of the worst kind. This is Carlos Castenada country!
So, let's say this guy does his 'shrooms, and his frame of reference changes such that he can't drive his car to the gathering because he can't find his keys ~not that he can't drive but he now sees his keys as being a piece of pavement or something.
He calls a cab and is taken to the event and he shoots up all and sundry.
Despite the fact a couple of witnesses said "he was firing randomly" he managed to kill 6 and wound 13 with just 20 rounds.
That's not random. It's well within the range of what's been demonstrated to be possible under the influence of hallucinogenics.
I will defer to greater knowledge. However, I have some experience shooting, and I know consistently hitting a target at 350 yards is not easy, even when you aren’t hopped up on something.
The type of person who consistently uses drugs is a good deal less likely, IMO, to have the self-discipline to develop the necessary skill.