Does that suggest I'd be better off working in batches of eight?
Yes, definitely.
I have a 4 core, 8 thread processor and have tested again the speed of rebuilding 4 and 8 raw image thumbs.
Although with Multi Threading Preferences for image processing set to Auto it was quicker with 8 images that doing two separate sets of 4. I got a much better result when I set Multi Threading Preferences for image processing to 8 threads. It was approximately 30% quicker than two sets of 4 separately.
Of course, there were no other programs open or doing stuff during the timings.
Try doing the timing yourself but use images without filters and any extra refinement to get a true comparison.
Terry