Working as intended, swiftcast only instantly casts, it does not instantly action. Holy has a longer action time the it's cast. Swiftcast allows you to release the orb immediately (no cast time) but the orb doesn't do anything until it pops, at that point you see the flying numbers. It has the greatest time difference between the 2 I am aware of making it the most noticeable. If you watch closely your stone spells do this also with a much smaller difference.