What does "High Performance" Storage Area Network (SAN) in the AMQ 7 doc mean?

Issue

Section 21.2. Journal Sharing and High Availability of the document states

This style of high availability differs from data replication in that it requires a shared file system which is accessible by both the master and slave nodes. 
Typically this is some kind of high performance Storage Area Network (SAN). It is not recommend you use Network Attached Storage (NAS).

What does Red Hat mean by high performance SAN in the above statement?

Ref: https://access.redhat.com/documentation/en-us/red_hat_amq/7.1/html-single/using_amq_broker/index#shared_store