Thanks aRtBee... at some point soon I will try logging some more formal stats on what I'm getting.

I was referring, largely, to just using the Background Render option in fact there... which does get me somewhat more CPU priority (and therefore a faster render output) seemingly, in general, than QM.

In fact, for a scene with less content (memory consumption) the CPU can seem to stay at 75% or more (overall usage) for the whole render process.

So the issue of it dropping down to just around the 25% mark for the IDL and Ray Trace / Render portions, seems to be linked to a scene being larger (e.g. memory and virtual memory each sitting around the 3 to 4 gigabyte mark)

I'm just unclear I guess as to why this would be the case for the first and last passes, but not for the interim SSS pass...

...anyway, entirely off-topic I think for this thread... so please ignore... I'll post separately somewhere in due course I expect... although I should try the SP2.1 update first really, in case there's a difference ;-)