I have always presumed that the settings for render and gui
  priority were only in affect when used in a multi tasking
  enviroment. That is to say if I have Pov rendering a scene
  and want to run other programs, with Pov running in the
  background, then I can use these setting to set the level
  of cpu and gui priorities I am willing to give each program.
  If Pov is the only program running then these settings are
  meaningless and all of my systems resources are dedicated to
  the task. I can't imagine that having the gui priority set
  to highest, with Pov the only program running, will greatly
  if at all, impact the render time of a scene. Especialy if
  that scene is going to take a long time to reneder anyway.

  Is there fault in my understanding of these setting and if
  so what ?
 

Ken Tyler