I got several deaths from monolith 70+ primeval dragons, I asked devs if the speed modifiers affect his one-shot explosions, cause all times that i died he INSTA cast his explosions…so devs told me that its not supposed to happen and its probably a texture bug not showing his circle. SO i made a build that can tajke his explosion even on monolith 70+. and today confirmed that ENRAGE mode does increase his casting speed. First half of fight i saw circles and could walk out, as soon as he went below 50% he went full pepega and cast INSTANTLY explosion with frequency 1 in 1-2 seconds. I kiled him in the end 
but please look into this thing. Im pretty sure 90% of builds will die to it very fast if not one-shotted.