MSexchangeAB/nspi rpc requests average latency very high

Question

Exchange 2010 SP2 RU3 mostly with one SP3 RU8 v2 Hub/CAS server.  All client access is being routed to the SP3 RU8 v2 CAS server.  I've noticed that the MSExchangeAB/nspi rpc requests average latency counter grows to extremely high numbers like
14K. When that happens, Outlook clients start to show performance issues and lock ups.  Then I restart the Address Book service on the CAS server, and it is back to normal for a while and then will eventually grow again and do the same thing.
Is there anything I need to look at specifically to see what's going on.  I think this has been going on for quite some time, like a year, but I haven't been able to narrow it down very well until now.
Any suggestions would be appreciated.
Thanks,
Andrew

Answer 1

In addition to moving processing of incoming Outlook Mailbox connections to the Client Access server, in Exchange 2010, directory access
is also handled by the Client Access server.
when an Outlook client makes a request of the Client Access server, it results in one of two possible actions.
If the user's mailbox is on an Exchange 2010 Mailbox server, then either the request is handled by a Client Access server in the current Active Directory site, or if the user’s mailbox is in
a different Active Directory site, the request is proxied to the destination Active Directory site.
If the user's mailbox is on a legacy Exchange Mailbox server, the directory request is referred to the user's Mailbox server. Legacy Mailbox servers can't communicate directly with Exchange
2010 Client Access servers for directory information.
You might have to check the health and replication across DC's in all sites. seems like there might be a possibility of latency from/to a GC which is being used by Outlook
for Referral.
https://technet.microsoft.com/en-us/library/ee332346(v=exchg.141).aspx
https://technet.microsoft.com/en-us/library/ee332317(v=exchg.141).aspx
http://blogs.technet.com/b/exchange/archive/2010/05/20/3409978.aspx
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