The fog on the Tyne is all mine.

Seriously, people were having problems with Deathground because of the lighting, but has anyone experienced slow down from using the light setting Fog?
I've an underground sector on my map I want to be pitch black, but the only way to do this is to set the time of day to -1. Which means the surface will be pitch black as well which is no good as I want it to be daylight up above. So I thought about using fog to obscure the area, but does this cause any problems for people? Bearing in mind I didn't have any problems with the lights in Deathground on my machine.