You need to be looking at seconds per read and seconds per write.  Those numbers will tell you how long the storage is taking to respond.

Performance Counters for SQL Server Related to iSCSI Disk Access
Am I losing any entropy when using KDFs, such as the ones in NIST Special Publication 800-108?

For example, can I derive 128-bit session keys from a (uniformly random) 128-bit master key?
