I tried poser 8 with more aggressive render settings and IDL enabled (not using 10.6.x) so that it would take more than a few seconds to try stuff before the render finished.  I found that firefox was a bit sluggish (not really serious, e.g. still able to edit this during a render).  the cmd-4 screenshots taken when poser was rendering were instantaneous, whether with 2 or 6 threads in poser.  thus I think there was something wrong with the implementation of P8XSR0 regarding 10.6.1 that wasn't wrong with the implementation of P8XSR0 regarding earlier OS.