The simple answer is that raytracing has a large default overhead.
That is, any scene with any object will take a *minimum* of time, which
is relatively large.

  However, while the rendering time of a 3D card increases linearly with
the number of polygons, raytracing time grows about logarithmically
(supposing we have some bounding box hierarchy).
  That is, rendering 20000 triangles with a 3D card will take roughly twice
the time required to render 10000 triangles. However, raytracing 20000
spheres takes only some percents more than raytracing 10000 spheres
(you can try it with POV-Ray if you want).

-- 
plane{-x+y,-1pigment{bozo color_map{[0rgb x][1rgb x+y]}turbulence 1}}
sphere{0,2pigment{rgbt 1}interior{media{emission 1density{spherical
density_map{[0rgb 0][.5rgb<1,.5>][1rgb 1]}turbulence.9}}}scale
<1,1,3>hollow}text{ttf"timrom""Warp".1,0translate<-1,-.1,2>}//  - Warp -

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