Thats hard to do and can't be done in Blitz3D as you would need to exchange the scenegraph.
But one way to speed it really up is creating boxes around zones you count as one unit (like rooms in a 3D shooter) and make the cube alpha 0 so it is not drawn but still used for occlusion. (don't use hide, this would disable it)
Then you parent all objects within the cube to this cube. This way, the whole content of the cube can be ignored on "rendering tests" if the cube does not need to be drawn.
*if you fill up the whole 3d room with cubes, then this would be like a static octtree system*
But one way to speed it really up is creating boxes around zones you count as one unit (like rooms in a 3D shooter) and make the cube alpha 0 so it is not drawn but still used for occlusion. (don't use hide, this would disable it)
Then you parent all objects within the cube to this cube. This way, the whole content of the cube can be ignored on "rendering tests" if the cube does not need to be drawn.
*if you fill up the whole 3d room with cubes, then this would be like a static octtree system*