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It's cutting up your map into vis leaves which tell the engine what needs to be rendered at any given time. If you run on fast vis it doesn't make multiple vis leaves so your entire map is rendered at all times which means much worse frame rates. It is absolutely necessary to run before a final release, but you can run fast to take a look at how it's going with the only side effect other than a poor frame rate being that water won't render properly.
Well, just a little comparison between a map that was rendered with fast render and one with normal:
Fast renderNormal render
Personally, I don't see much difference, but I'm willing to bet some one will prove me wrong.
EDIT: I see crazypip beat me to it, so I guess the issue is frame rate. I guess is true, though on both I got 60 FPS.
Because, when you run VVIS on Fast and you run VRAD (Fast or Normal, Either Works...) assuming you didn't leak, VRAD will do the "BuildFaceLights process, then the next thing it will do is the BuildVisLeafs process...
I'm in the process of compiling a map right now (Fast VVIS and Fast VRAD) so in a few minutes, when it finishes, I'll go into game on said map, then I'll do sv_cheats 1 and mat_wireframe 1 and you'll see objects being hidden behind stuff and on the compile log I'll copy where it says how many leafs there are... All without the Num Portal thing wasting my life.
Oh, and water works for the most part (it may render wrong tho) so long as you run Fast VVIS... You don't need VRAD to get water and you don't need the Num Portal to run for water (though it can --but doesn't always -- look a little messed up in this situation.)
You said that:
"It's cutting up your map into vis leaves which tell the engine what needs to be rendered at any given time. If you run on fast vis it doesn't make multiple vis leaves so your entire map is rendered at all times which means much worse frame rates."
Okay, well, I just ran my map on Fast VVIS meaning no NumPortal thing going on...
In game, vis leafs worked and successfully kept my comp from rendering portions of my map that were not visible. Therefore, Normal VVIS does NOT cut up Vis Leafs... That's done by VRAD. So, I ask you, what goes on when you run VVIS in Normal instead of fast? We both know it runs the Num Portal thing, but would you mind telling me exactly what that is//does? (Assuming you know...)
VVIS - This tool creates the visibility matrix for the map based on the level's geometry. This matrix is what determines which polygons the player can see, might see, and can't see from any given point within the level in game. It is critical that this tool be run on every .bsp you create. If it is not run then the entire level will be visible all the time which would, in all but the simplest of box maps, make the map unplayable due to video-lag. In-depth information about VIS can be found at visibility determination.
VRAD - The RAD tool, or Radiosity tool, is responsible for generating and applying all lighting effects in a level. Everything from entity lights and the "sky" down to the lowly texture light has to be handled by this tool. A .bsp that has not had RAD run on it will appear pitch black in game or full-bright depending on video-mode and console-settings. This tool is normally the one responsible for slowing down compiles most of the time as it has to process huge amounts of data in the completion of its task. In-depth information about RAD can be found at RAD (technical).
VRAD is strictly LIGHTING, nothing more, continuing to argue the point is not worth your time. VVIS run on fast only does quick sloppy calculations on your levels breakup, while this will grab simple areas fine the more complex rooms/geometry will just be slapped into one large leaf rather than broken down into the finer leafs that you require via a Full compile which does more precise calculations and checks each one while it goes. Now, if you've done some very good optimization and tools use and all your rooms are perfect squares then a fast compile will give 90% of what a normal full compile would. But still not perfect.
So rather than complain about it taking forever and that there is no point to it when fast does the same thing, take the time to learn about simple optimizations you can do, there are some great tutorials and an entire guide over on interlopers.