As a test I sent a lot of badly formatted email into some test mailboxes (just 1000-2000 messages). Mostly they weren't word-wrapped at all.
When opening these within the web interface I see delays of upto 3 minutes getting a response however. If I delete the messages and send the same text but wrapped properly then the response goes back to normal.
Accessing them from a normal IMAP client seems ok however.
Has anyone else seen this kind of behaviour with poorly formatted mails?