Have you checked the vmkernel.log on the host to identify any issues with connectivity to the RDMs?
Also do you have the RDS's setup as described in the Setup for Failover Clustering documentation? https://pubs.vmware.com/vsphere-60/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-60-setup-...
What could be the cause of the delay is that the RDMs used for MSCS are not set to perennialy reserved. During a LUN scan the ESX host tries to claim all the available LUNs presented to the host. If the LUN cannot be claimed because the LUN is already claimed this action can take a very long time. ESXi/ESX hosts with visibility to RDM LUNs being used by MSCS nodes with RDMs may take a long time t...
