@mafon2: Sealed Vs Leaking
About «sealing the map». I'm not so familiar with the concept
Make a simple box room out of six brushes that form a hollow cube, and stick an info_player_start inside that box. Make sure that there are no gaps where the brushes meet. Now compile the map and run it. When you noclip outside the box, you'll notice you don't see the backs of the brushes -- instead, you keep seeing the inside of the box even as you move into the void. This map is sealed: the brushes completely cut off the playable area from the void (i.e. endless empty space) around it.
Now go back into the editor and move or resize one of the brushes a little so that there is a small gap between it and the other brushes. If you compile and load that map and noclip into the void, you'll notice that you can now see the backs of the brushes too: you'll see the entire structure you've made in the editor in-game, just sitting there in the void. This map is unsealed, or leaking.
That's the basic idea, but it applies regardless of how complexly shaped your map is -- you should always make sure that it is sealed off from the void.
Another way of thinking about it is to imagine that the void is made up of water, and you want to make your map watertight, so there should be no tiny spaces for the void/water to "leak" into the map.
When your map is unsealed, the engine has draw tonnes of unnecessary stuff (the outside of the map that the player should never see), and you cannot run vis
-- and when a map is unvised, the engine likewise has to draw tonnes lots of unnecessary stuff (i.e. the parts of the map beyond what the player should be able to see at any given point).
Note that you cannot use brushes with a water/slime/lava texture to seal a map, so e.g. if the floor of room is made of slime and beyond that is just void, you'll need to stick another world brush underneath the slime brush to seal the map. The same applies to func_detail brushes, if I recall correctly. Sky brushes are fine, though.