Uhm... No, it can't be that. I continued the run a bit and the effect is instant. They cast and BAM, half the party is dead and the rest is badly damaged. I think they simply do a lot of damage all of a sudden, at a certain point, when they cast. Dunno.
Not complaining. They are strong and I'm happy my teams can't reach infinite god mode every time. I just don't understand what's making them that strong.