Cycles Render Speed Question

I have the (probably) most basic setup for cycles rendering: A plane, a cube, a sun and a camera; no textures, no materials, no additional layers, no nothing more. Rendering with 128 samples takes +/- 48 seconds. Is this normal?
I have no GPU but I went through pretty much all help available in Youtube etc. and basically the 48 secs stay the same, whatever I change in parameters. Just to name a few changes I tried:

  • tile size
  • causistics off
  • bounces to 3/0
  • sunlight max bounces turned down
    The only effective change I can generate is switching from “path tracing” to “branched path tracing”, where the standard settings (AA samples render = 4) works quicker, but it looks very bad.

I do not give you the details of my computer because my central question is: Why does rendering time not change when I modify parameters that are described to have an influence?

Many thanks upfront to all you smart guys!