Statistics:

  1589 octree nodes (non-leaf),
  5363 octree volumes (leaf nodes),
  889171 (21966848) voxels

 
That's 889,171 non-air voxels and 21,966,848 voxels if air voxels in volumes are counted!  No wonder the frame-rate struggles.  Once I add level-of-detail optimized rendering the raymarching will run faster for distant volumes.