[gaelow]

It's arithmetic, not calculus. In both your performance depends on three factors: technique, practice, capacity. I don't know about capacity, but practising and learning calculus techniques is not the same as practising and learning arithmetic techniques. In fact if you wan't to get good at calculus it is probably counter-productive and inneficient to waste time on solving arithmetic operations "by hand" when using a calculator will be much faster and help you keep focused on the calculus problem at hand.