Quote - > Quote - I do have texture filtering set to "none" on some Image map nodes... set to "quality" on others. What are the implications of that? 

Disabling texture filtering dramatically reduces the efficiency of the texture cache. When using large textures, this can result in a lot of disk access from the renderer, since it has to load new texture data constantly. Since only one thread at a time can access the disk, render threads can end up spending most of their time waiting until it's their turn to read texture data.

@Stewer

Well I've set the unfiltered image_map textures to be filtered to either "crisp" or "quality"... based on your point about disk access, for texture reference, being a likely source of threading bottlenecking, I've also tried optimising various other textures (e.g. by shrinking the pixel sizes of texture image files for anything not in close up) and that large scene is now rendering much faster, with much higher CPU utilisation.

It's still taking a long time... but its another pretty stupidly complex scene I'm doing I guess. Having those seven environment spheres enclosed within the main environment sphere enveloping the scene, probably won't be helping... LOL.

Especially if texture image referencing from disk is the multi-threading nemesis.

So optimising the maps for those, has helped a lot too by the looks of it... as well as making sure that texture filtering was on everywhere ;-)

Many thanks for all the advice relative to my off topic interjection!