Why is putting a giant box around your map bad? What could go wrong?

Because it messes up VVIS's visleaf calculations, making your map much more laggy. (The game must compute and determine what parts of the map can be seen from every place outside your map, which would often cause it to render large amounts of your map that are actually obscured by walls or similar things.) See Interloper's optimisation tutorials or the VDC article on optimisation.

With Old Aperture maps you would need to "box" the map to seal the sphere model from the void, this is basically the only exception to boxing maps.

What TeamSpen said. When it 'cuts up' your map into visleaves, it will also cut up these open areas. Instead of it stopping at the edges of your geometry, some lines will keep going into your box. These lines intersect outside your map, creating hundreds and sometimes even thousands of unnecessary visleaves. This will greatly extend your compile time and will probably make your map run slower.

If you do need to box off a map, for example in the Wheatley theme, you can use func_visclusters to group leaves and reduce your compile time.

The forumla for total calculations is:

Numportals * (Numportals - 1)

If you had 2500 numportals, which can happen in a boxy map, total calculations would be 2500*2499 = 6,247,500 calculations!

[Plus, the number of calculations it goes up by when you add 1 numportal is Numportals + (Numportals - 2). So if it went from 2500 to 2501, it would have to do 2501 + 2499 = 5000 more calculations!]

The compiler does about 3900 calculations per second on average, so that means it would take 1601 seconds to do VVIS - that's 26 minutes and 41 seconds!

Compare that to a 500 numportal map, which would have only 24,500 calculations, taking the compiler about 6 seconds to do VVIS. In actual fact it's a lot faster than that, the 3900 figure isn't perfectly accurate.

The moral of the story? Keep your numportals down! And the best way to do that? Don't box your map!

Thanks alot guys!