Very slow with imap

Question

I have just got a Macbook Air 2011 with Mountain Lion. Everything goes fine, except Mail. Normally, it's working. But when I read imap messages, even the message size is less than 1MB, Mail is very slow, even frozen for a while. CPU will be used much, I can hear the fan working at max speed, I assume.
Downloading messages is fine, it is just making me crazy when clicking to imap messages. Any idea to fix it please?
Thanks.

Answer 1

The MimeMessage constructor you're using needs to copy the entire message from the server.  It uses the Message.writeTo method, writing the message content into a pipe that is read by the constructor.  It was a bug in previous releases that the writeTo method wasn't fetching the data in chunks, as it does in other cases.  Setting partialfetch=false will revert to the old behavior for writeTo, and for all other accesses of the message content.
It looks like both 1.4.4 and 1.5.1 are fetching the message content using a single FETCH command when you set partialfetch=false, according to your message.  If they're both sending the same commands to the server, I can't explain why it would be fast in one case and slow in the other.
I did similar testing using my Exchange 2010 server.  Fetching a similar size message took less than 20 seconds.  Setting partialfetch=false brought it down to less than 5 seconds.  Something's clearly wrong in your environment, but I don't know what.
Finally, I'm sure you understand that you only need to use that MimeMessage copy constructor in special circumstances, and you should probably avoid using it unless it's absolutely necessary.  Even in the best case it's wasting time and memory.