Do not overuse the aim, meaning do not walk around aiming your gun everywhere. The problem becomes blatant when you walk into an open space and need to examine your surroundings quickly, so as not to get shot. Your aim speed is very slow. Instead, use the camera to look around quickly. Also, the direction in which you point your camera is where your character will aim his weapon when you press [Aim]. This is much faster than manually aiming. Finally, do not spray and pray.