An open map? Source engine, which is funnily a Quake engine on steroids (a bit of exaggeration but still), inherited the same limitations of its parents in terms of visibility calculations: BSP and PVS. This fact makes Source, as was Quake engine before, more suitable to rooms and hallways separated by portals where the BSP shines in all its glory. Inheritably, Source does not like large open maps where the PVS is of considerable size and the over-rendering is a real issue. If you