If you use branched path tracing, the render will clear at a lower -often much lower- overall pixel sample number as compared to a rigid (non-BPT) number of pixel samples. BPT adds more rays/samples only where it is needed. E.g., the crystal goblet would get extra attention, while the paneled wall and tiled floor just get the set number of pixel samples.

You mentioned your promo size was 1500x1500 and bucket size is 126. For Firefly, buckets are most time/memory efficient if you use an integer power of 2 (i.e., 2, 4, 8, 16, 32, 64, 128, 256...). I don't know if Superfly has any such preference.

For either render engine, you waste render time if you use a bucket size which is not a factor of both the horizontal and the vertical pixel dimension of the render. So for example, for a 1500x1500 pixel Superfly (CPU) render, I would use a bucket of 15, 25, 30, or 50.

For Firefly, I'd set the render dimensions to 1600x1600 pixels with a 32-pixel bucket, or maybe 1200x1200 (the forum seems to reduce anything larger anyway) with a 16-pixel bucket.

I'd reset the number of render threads (in general preferences) to 8 for a H/T quad-core processor; you can still do other work while rendering. If the occasional response delay bothers you, you could lower Poser's (FFRender64) priority.

What are the chances of a postal packet with a few processors inside getting past Brazil's customs without huge taxes? Could you afford to scrounge up an (obsolete) motherboard with two 1366 sockets? I have HyperThreaded hex-core Xeons on my parts shelf.

Those are Westmere series Xeons, no longer in production. Any model number beginning with E (E5639, e.g.) is tuned for economy. Westmere Xeon processors beginning with L (L5520, e.g.) are a compromise between economy and performance. Westmere Xeons labeled X (X5650, e.g.) are tuned for performance.

X5650, X5660, X5670, and X5675 will run on motherboards with 1366 sockets rated for 95 Watts; the X5680 and X5690 require 1366 sockets rated for 130 Watts. The bare motherboard pictured in my earlier post is a SuperMicro X8D... model; ATX size, the dual 1366 CPU sockets are rated for the 130W Xeons. Letters/numbers after "X8D" indicate various combinations of RAM slots and expansion card (PCIe, etc.) slots.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5