We have an alomst identical issue with ESXi 5.1 & Nimble cs220's - slow iscsi performance and high latency 400ms +
We spent past 48 hrs doing a loop between Nimble, Cisco and finally vmware. Each saying it was not their issue.
Right now we are looking at a Physical windows 2008 server mounted iscsi to a nimble volume. As we see blazing performance.
If you have any ideas we would love to know.
Thx