Wow that explains a lot.. performance on Development has be awful for me lately and I have been unable to figure out why, this might be related.
My setup is 2 400GB drives in RAID0 using dmraid with LVM ontop.
It's a while since it tried dmraid instead of mdraid, does it use a UUID as the PV name for /dev/mapper? I suppose pvdisplay would show, then you could try hdparm on the PV under LVM compared to the LV above it.