I created a "room" to serve as a pond on a map. The room is about 60x60 squares and has 24 sides. Editing the room (dragging vertices, splitting walls, etc.) was no sweat when all of the walls were straight lines, but then I started converting all of the straight walls into curves. After about the 5th wall, things started to really slow down. But the time I got to the 20th, it would take the program about 30 seconds to accept any edit to the room. It seemed as if dungeonfog was reassessing all of the curved walls of the room every time I made a change.
For now, I will use the brush feature to make my water, but it would be nice to have the functionality of the room tool for making large curvey shapes.