Ok - I think this is how it works. 100% haste will cut the speed of the spell in half, then you take the left over 21.68% and multiply the cast speed by it, and then subract it from half the cast speed - so this is how it works i think:
Half of 2.5 = 1.25 seconds = 100% of the 121.68% haste
so what's left is 21.68%, 2.5x0.2168 =0.542
So 2.5-1.25-0.542 = 0.708 seconds
Which means at level 10, a holy light would go off at a speed of 0.71 seconds...O.O