Just wondering if you had the chance of testing the network speed with iperf from host to host yet?
This is like the first thing to do while doing a full debug upside-down. No matter what disks you have but note that each and one of them has a maximum IOps limit which sometimes you can hit in a way. as already suggested by others in the past you can use a binary search dividing the issue one part at a time network.. disk.. controller.. driver.. software(since the software is proven to work).

Eliezer