My point is that the "set selected messages of XXX" command told to Mail (not to System Events) does not work reliably.
Sorry for the confusion. I was under the impression that we were both talking about GUI Scripting. I agree with you that the *“selected messages”* property doesn't seem to work properly (as I pointed out in my [reply to Camelot|http://discussions.apple.com/message.jspa?messageID=11493860#11493860]) .
On the other hand, I don't understand yet why GUI scripts are so slow on your computer, whereas they work very well on mine. Do you have a huge number of messages in your message viewer?