One mistake that I consistently made in the past is aiming at head level, but my crosshair would be right on the corner so that if anyone swung the angle I had to flick a little in order to hit the headshot. This little flick is NOT a normal part of crosshair placement- ideally, you should just be able to shoot without even having to think about flicking or moving your mouse. Place your crosshair a certain distance away from a corner so that if someone were to wide swing the angle, you only have to shoot to hit the head- no flicking involved. Factor in your reaction time and elevation to figure out how far away from the corner your crosshair should be. Especially if your reaction time is incredibly slow, being able to guess exactly where to place your crosshair without having to spend time flicking to heads is a super huge benefit.