Potentially seeing through walls: a performance issue

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I know the engine is fully capable of handling what my maps currently hold, so long as the user's vision is blocked from seeing some of the map. However, it would appear there is an issue where, though you (the actual user) cannot see through a wall, the game seems to think that the player can really see through it. This is the only logical conclusion i could up with, as this issue isn't making much sense to me, at least from what can be visually derived by me.

This issue has also caused me a be of a fright, knowing that a monster is locked up inside a closed building, while i am outside walking through the town and noticing my vision become blurry when i'm not even staring in the direction of the monster! Do you know how creepy that is!? (Temporarily disabling the monster made the blurry vision go away when staring down the same spot.)

Thinking back on this, i remember having an issue where if the user stared somewhere near the direction of a Script Area that is outside the building and the user is inside the building, the game would activate the callback function set by SetEntityPlayerLookAtCallback(). I didn't think much of it back then since i was able to fix the problem by adding more decorations around the door where the issue was detected.

As for my current issue, i was able to achieve expected performance by placing black planes in certain areas. However, this is proving to be impractical as this could not solve all performance issues for all the noticeable areas. I can only imagine what kind of performance issues i'll be having for my next map, which is expected to be about twice as big as my current map.

If anyone could provide some insight on this, it would be much appreciated. I will continue trying to figure out the main source of the issue in so far as i am willing to, but this is constantly proving to be futile!