Just a heads-up on a weird superfly 'gotcha': I ran into this while trying to get my new 'network render' Box to use more than one CPU thread. Then I found this PC had the same problem with CPU-only renders...

Took a while to spot that Poser's default superfly render parameters were okay, used Box' 16 threads or this PC's 8.

From there, I soon found the problem lay in the 'many-buckets' setting that suited my twin GPU cards until scene was just too complex, forcing renders with CPU-Only..

So, if switching to/from GPU / CPU, try not to set a CPU-only render more than ~64 'buckets' lest 'single track working' ensue...