Forced visibility, I assume would (override) your, or anyone playing your maps fog distance settings, and maybe rule out your separate zones in your map. What I mean by zones in your map is, if you put a zone portal between two separate parts of you map, if you force visibility, then maybe it makes the comp render the whole map, instead of just the room you are in, but could just as easily mean fog distance.
EDIT: OK I didn't notice that you said an Item's properties. What that is, is no matter what zones you have throughout your map, you can see that Item from any zone. Which to me would make that Item priority over everything else, then in turn slow the map...