Wow! Of course, that Parallel.For() loop helps too, eh? Two of them!
The Nurbs curves look very weird though, better to use the DeBrep ‘E’ (Edges). Or PLine (Polyline) with inverted ‘C’ (Closed) input.
P.S. This ‘out worldXY_bbox’ is interesting too?
BoundingBox pln_bbox = crv[i].GetBoundingBox(planes[j], out worldXY_bbox);
As we discussed in PMs, that returns actual box geometry, though the distinction is still fuzzy for me.