I believe it would be better if you would use skymap textures, since the giant cube way could spawn lots of portals. More portals -> more leafs -> larger BSP.
Allso, if the visibility checks would go wrong, the game would draw parts of the map you normally can’t see from the place you’re standing, but can be seen according to vis calculation. Drawing unseen parts (aka overdraw) == performance loss. If it would have serious amount of overdraw (read: it tries to ‘overdraw’ huge outside areas) you 'll get a huge performance decrease.
If you have problems regarding leaks, make sure you use enough detail brushes and check the solid hull.
NOTE: I could say some wrong things here, but probably someone will correct me in that case… :nag: