Ray of Darkness and Spell Casting Speed

I used Dark Ray with the casting time increased by about 40%. By default, Dark Ray deals 12 damage per second, so in theory it should kill the Goblin Archer with at least two uses. However, when I used Dark Ray in this state, it took 4.5 uses to kill the Goblin Archer. What is the relationship between Dark Ray, casting time, and damage ratio? It seems strange that increasing the casting time weakens it.