Full: For 1...8 seconds, spells cast by the target foe and all adjacent foes take 100% longer to cast. Spells targeting these foes that are cast by you or your allies take 50% less time to cast.
Concise: Also hexes adjacent foes (1...8 seconds). Doubles spell casting time. Spells cast by you or your allies on hexed foes have -50% casting time.
At 16 in Fast Casting, most 1/4 spells are so quick the character does not go into the animation of actually casting the spells which leads to the case of instant casting spells.