I guess other than the large mail boxes, these are some other factors:
- A large number of users on the BlackBerry Enterprise Server (for example, 300 or more).
- low network speed. During a mailbox scan, the BlackBerry Enterprise Server accesses the Microsoft Exchange Server where device users are located.
- Messaging server performance.
Reference Source: http://www.blackberry.com/knowledgec...nodeid=1177775