Comment 56 for bug 927515

I have 2 IMAP accounts configured, and noticed that the CPU hogging / hanging is definitely related to the number of messages in a mailbox. I haven't found a lower bound that causes the issue, but mailboxes with e.g. 20000 or more messages of cronspam were definitely part of the problem. Removing these messages and/or partitioning them e.g. chronologically may be a viable workaround.

Testing with "insanely large" mailboxes should definitely be part of finding the problem ;)