The following error comes up after about 30 sec after clicking on the mail collection containing large number of mailboxes (~ 2000): "[Microsoft][ODBC SQL Server Driver] Timeout expired". However when clicking on the Calendar collection of the same size it becomes available almost right away.
Migration Manager previously had SQL timeout parameters configurable via the registry and then hardcoded as 60 sec into the application itself. However at some point this concept was dropped and timeout removed. There is currently no SQL timeout exists within the console.
When clicking on the mail collection, along with the population of the list of mailboxes QMM is also populating switch status in the form of an icon displayed over the mailbox. Normally this information is retrieved from ADAM if "Refresh Switch Status" option is utilized. Once refreshed it's stored within Statistics Database and then always retrieved from there. In this particular case timeout errors disappeared after purging some of the old mailboxes statistics and therefore reducing the size of Statistics tables within SQL project database.