The way Blackberry server discovers the mailboxes for the BES users by using MAPI and rapping worker treats into the MAPI session. Worker treats are processes build into the BES server, to discover and establish state full connection to the hidden BES mailbox on the each user mailbox. Each BES server is capable of handling 100 worker treads and up to 2000 users.
Mailbox moved within the same server, from one mail store to another one, will break the BB connection for the user got moved, due to limitation build into the BES user mailbox discovery process. Because BES scans users mailboxes for changes in the Server DN (distinguish name).
Blackberry Server normally generates 3 to 4 times more MAPI traffic then a regular MAPI user. The BES is being used in conjunction with latency caused severe problems. Not also BES experience but also other aspect of the network will suffer as consequence of this problem.
The size of the mailbox is not what causes the latency but specifically the number of items in your Outlook folders. The number of open items results the same behavior in poor outlook experience the famous Christmas balloon "exchange is retrieving data from such exchange server" everyone's favorite message will appears from the outlook.