10 at a time: 49 images in 7min 4sec (8s 673ms per)
8 at a time: 49 images in 6min 48sec (8s 327ms per)
6 at a time: 49 images in 6min 44sec (8s 245ms per)
4 at a time: 49 images in 6min 44sec (8s 245ms per)
2 at a time: 49 images in 6min 43sec (8s 224ms per)
and finally
1 at a time: 49 images in 8min 1sec (9s 816ms per)
your suspicion is correct, though why 1 image at a time would take longer is a bit strange- perhaps there is some opportunity cost in going from one to the next if you have to wait for one to complete? It does look like 2 is optimial, but not by much.