Such as the null, clip, and sky textures are the same; as in they render differently, and should be used on surfaces that will not be seen [...]
null, clip and sky are completely different things. null
changes nothing about the solid, but removes the visible face from rendering. However, I believe having all null faces on a brush can cause unexpected behavior when the brush is part of an entity in some rare cases. clip
is used to block player movement. That's all it does. Importantly, however, clip does NOT stop leaks
. It ONLY blocks players. Other than that it counts as empty space. sky
, obviously, is used to display the sky box. Faces with sky are not rendered (like null) and cause the engine to draw the skybox in the direction of the face. The skybox is just a big box around the player, so it can block far-away objects, and if you are not careful, you will be able to see things beyond sky faces, like other rooms.
[...] to decrease load time and increase fps. To contradict that, others say that those textures don't actually decrease load time and increase fps.
clip has no such effect. null and sky can increase fps if used in place of normal textures, where appropriate.
Furthermore, I read somewhere that putting [for example] a 128x128 texture on a bigger surface will cause that surface to render in 128x128 squares thus increasing load time and decreasing fps.
This is not true. However, the scale of the texture will affect the lightmap/patch density, which could be disastrous. This can be adjusted with the -chop option in hlrad.
[...] putting a 'complex' object adjacent to a 'simple' surface will cause that surface to render in as many sides as the complex object [...]
This is true. Avoid this problem by making the complex object a func_wall, for example. Then remember that brush entities will not stop leaks and build accordingly.
Also keep in mind that hlbsp breaks up and organizes all the open space
in your map into convex pieces. If you make the open space in your map really complex, hlbsp has to do more work and so does the game engine.
[...] giant rock that you are only going to see say 5 out of 9 faces, would it be more efficient to hollow it and delete the four faces and null the back of the other five?
DO NOT hollow it. hollowing will just add a lot of faces. If you are lucky they will be merged back together by hlcsg/hlbsp. But don't tempt it. Feel free to put null on any face that need never be seen. It won't hurt. It may not help much either, but it won't hurt.