It sounds to me like you should put your interiors in their own level and compile them as map entities. When the player comes in the door or gets within range X you load the map entity or set invisible = off for it.

I don't know that using fog will increase your frame rate. It is more of a fade out to the clip plane so you don't have entities pop in and out at the edge of the clip plane.

You could also break up your world into say 5000x5000 quant map entities and load them in and out as outlined above.