That is partially what I meant actually. To makes voids work as an optimization in a non-jarring way (in an outdoor level), you have to slope the ceilings around it as smooth as possible. This is much easier to do with equation slopes and vertices.Due to unoptimizations in the game, blocking sightlines with ceilings does not actually work to solve lag. To block sightlines, you will want to outright delete sectors, making a "void" in the map of sorts.
Why slope the ceilings, if you have them high above the surface you end up with high pillars in the middle of the level, which may not be what the theme called for. Sloping them can reduce the height of the walls dramatically and ensure flying characters do not look at that part of the level from above.