Some answers from the funtion owners:
>Does ippKDTBuildSimple type have any purpose?
Simple kd-tree building algorithm always returns a trivial tree with all triangles associated with a single leaf node at the kd-tree root. It was designed for testing and benchmarking purposes.
>Could you please describe what QoS parameter does?
Advanced kd-tree building algorithm (ippKDTBuildPureSAH) is based on recursive subspace subdivision according to surface area heuristic (SAH). Subdivision of a leaf node is made if the best SAH cost after split is less than original SAH cost of this node plus cost of split. The cost of split depends on QoS, the higher QoS, the lower the "cost of split" thus kd-trees with higher QoS tends to be deeper but more efficient in general.
>Perhaps it's a worst case scenario for kdtree acceleration? Lots of small-long triangles spread around.
Small-long triangles should not be a problem for our kd-tree building algorithm because SAH-based optimization takes into the account internal nodes from triangles / split planes intersections. On the other hand, you can try to make Delaunay triangulation to see if its true or not.
Kd-tree is good in helping you to find the closest intersection, but if there is no intersection at all for many rays, you still need to traverse the whole volume without early termination. In any cases, large number of internal holes in the displayed structure makes it different to render using any spatial acceleration structure. Its even worse, because rays are traversed in packets, so even if one of the rays do not hit the surface, the whole packet continues to traverse, decreasing the efficiency of intersectors. It might help to decrease the packet size for such scenes but in general it can make things worse.
>Do you have any recommendations to pinpoint the source of slowness in this scene?
One additional recommendation is to scale the model to fit the unit cube and see if it helps to increase the performance. If your model is too large, several internal heuristics might not work well.