Dunno if all the cop reality shows on tv are accurate, but I see them burn a whole mag as fast as possible. It's hard to be accurate doing that. Hurry slowly.
Every now and then, though, you see some of them do some really excellent shooting on those shows.
Things like firing out the side window of their patrol car at freeway speeds at a tire on a car they are chasing and taking out that tire in one shot or the one who was fired on by the driver of a car he had stopped, and recovered quickly enough to put most of the magazine load through about a four inch circle in the rear window as the car was rapidly taking off (hitting the driver).
Some can fire extremely well, even under great stress. Others apparently couldn't hit a barn while standing inside it. (LA shootout against armored bank robbers comes to mind - with IIRC 672 rounds expended... All that was needed was two accurate head shots.)
Sadly, with today's high capacity pistols, as opposed to yesterday's wheel guns, there appears to be less concern with a few well placed shots, and more a "spray and pray" attitude in hitting the target(s).
Even with a target at no more than arms length, there is a tendency to empty magazines, as NYC officers did when shooting a man reaching for his ID 41 times at close quarters.
Anyway, just like the general gun-owning public, there are LEOs who are excellent shots and those who are abysmal. HOWEVER, being as it is part of their job description, they should on average be much better than they are IMHO.