I am also very interested in this.
There are other causes of delays when going to UM also, mainly the size of the inbox, sent items, and deleted items of each Unity subscriber. Unity has to index, as well as any email reader like outlook and that will cause delays.
While one subscriber's inbox/mailbox is indexing, it seems that everyone on the Unity system is delayed until that subscriber is freed up. Even if they have a smaill inbox. I have tested this.
We are looking as size and count.
I have documentation from cisco if you want it but you will not be happy about it.
rlp