is there a time lag?
 

is there a great deal of time lag in accessing data from SAN or NAS or any other data center storage device? if so,why?

There shouldn't be a visible lag of more then a few miliseconds on a local area network, unless there is so much traffic the switch gets congested. When you access a NAS from outside of the network you will then have the issues of general internet traffic and therefore maybe suffer some delay latency.

If you are going over an external network then it can be many factors. But over LAN it should be fine...

In house on your network LAN there shouldn't be any issues with lag time. The only problems I found once was when a few people were using the network to pass large personal files back and forth.