Problem solved: Was excessive buckets for CPU-only render !!!

https://www.renderosity.com/rr/mod/forumpro/?thread_id=2958942

64 vols +1024 buckets Settings for my twin GPU cards do not suit a CPU-only render.

Break-point seems to be 128 volumes + 128 buckets giving ~90+ % CPU.

64+128 ~100%, 64+256 ~ 75%.

with 128+128, that six-up scene remote renders ~10x faster, ½ hr instead 5½ hrs...

Yay !!