That's queer, I haven't noticed the Voxel map being any more or less laggy than Rei's, and honestly on SSP, the whole being able to properly navigate caves and hell thing is pretty damn useful. Even with waypoints, there are times I just get plain lost in caves because Minecraft likes to give me an asston of labyrinthine mazes. Then again, this is a total topic derail soooooo...sermon over.
@OP: Is there a way you could make a Mystcraft age that's nothing but swampland, locked at permanent night at the new moon? I think that's possible in the newest versions of it, and while it may be a bit inconvenient, it'd solve the slime issue.