Test out the most recent version of Zodiac. The boss uses DrawTile to set up long jets of flame at one point. In Beta 718, it was running full speed. In 735, it slows down dramatically when drawtile is enabled.
Massive slowdown (Drawtile) since Beta 718
Are you sure it's drawtile? Take a look through Allegro log next time you play. I was getting this and had no drawtiles going.
Your script is probably trying to dereference an invalid NPC pointer. I added temporary code to print extra debugging messages to allegro.log when you do so, to help find a bug.