I think we'll need to look at the individual issues. Where exactly are things rendered, and see if we can fix them. Chances are, it will be tricky to fix as a lot of your corridors line up, meaning visleafs are seen at a far distance.
The blue lines in the images I've shown are not actually Visleafs, but the PORTALS between them. Each rectangular blue area is a portal between visleafs. During the calculation, each portal checks if it has line of sight with all other portals. The more portals, the longer this takes. This article explains it well: www.optimization.interlopers.net/index.php?chapter=visleafs
What do you mean FastVis=False? That sounds like something the expert compile would use.. so it's the equivalent of VIS set to Normal on the normal compile mode (?) And yes, I set VIS on Normal, otherwise the VVIS process wouldn't take 5 minutes..