I think I got my answer, I was simply dd if=/dev/hdiskX ... and I should have done dd if=/dev/rhdiskX ... using the raw device instead of the block device increased the instate result significantly !...
I have fresh new installed VIO 2.2.3.70 on a p710, 3 physical SAS disks, rootvg on hdisk0
and 3 VIO clients through vscsi, AIX7.1tl4 AIX6.1tl9 RHEL6.5ppc, each lpar has its rootvg...