Steve, are you building multithreaded? What threading backend? Version of VTK?
We had significantly sped up vtkGeometryFilter (with threading), but more recently Spires has worked over the vtkDataSetSurfaceFilter (also with threading) and I was under the impression that vtkDataSetSurfaceFilter ended up being faster. So I am confused by the significant difference in your case. Hence why the VTK version number question…