Found the offending effect: It's the vapor coming from a cauldron inside the castle.
The framedrop in this particular level comes from the entities (models) camping around: When you destroy/kill whatever you see, it starts to alleviate.
So no problem (?) -> just a matter of cpu cycles.
Edit: Why this effect is working outside the room/area it belongs ? I did a new wire/line drawing mode & combined with noclip you see the effect some walls before all other models are drawn....