- the algorithm was last used in OpenFOAM-2.4, after which it was
  replaced with a FaceCellWave version.

  Whereas the original (2.4.x) version exhibited performance
  degradation on very large meshes (with explicit constraints), the
  FaceCellWave version exhibited performance issues with large numbers
  of blocked faces.

  With large numbers of blocked faces, the FaceCellWave regionSplit
  could take between 10 to 100 times longer due to the slow
  propagation speed through blocked faces.

  The 2.4 regionSplit has been revamped to avoid local memory
  allocations, which appears to have been the source of the original
  performance issues on large meshes.

  For additional performance, intermediate renumbering is also avoided
  during the consolidation of regions over processor domains.