It has nothing to do with a "BSP-Tree", but otherwise Ventilators explanation goes into the right direction.

A6 renders level blocks not as a whole mesh but as a stream of single polygons. This has no big disadvantage with normal surfaces, but becomes slow when using effects or shaders because the effect is then initialized for every single polygon.

For this reason the new kernel we're currently working on does not render polygon streams anymore, but meshes.