The guns were quite accurate. They had rifling etc long before the Maxim gun.
Being a top grade British rifleman required hitting a 3 foot wide target at 900 yards or something. That’s pretty fucking good without glass optics.
They were slowish to fire, but they had paper cartridges that made it not too slow. Lower casualty rates probably have more to do with soldiers not being brainwashed yet, lots of people didn’t actually shoot to kill. Compare the casualty rates of the colonial campaigns where soldiers didn’t consider their enemy human.