What is com.apple.mail.plist and why was it slowing Mail down? I fixed an annoying problem with Mail 6.2 (I'm on ML 1.8.2), where Mail was slow and a pain to use.  I am on a SSD so everything was jerky while typing, sending mail out, and it would take ~1 second to display the text of a message when I scrolled up/down the list.  On a HDD I presume the problem would have been much more bothersome.

I looked at Mail slow under ML/ Apple forum and someone mentioned to look in console for errors such as deny file-read-data /Library/Preferences/com.apple.mail.plist, and to delete such plist in case.  I did have such message, so I went on to delete the file (and a file of the same name in ~/Library/Preferences/).  Now Mail works fine, no slowness, no jerkiness.  So the question is, what the hell is com.apple.mail.plist, what was doing in my laptop and why Mail seems to be much happier without it.

FYI my laptop had a install from scratch of ML + backup restored through Time Machine.  In addition I restarted Mail after deleting com.apple.mail.plist and all my personal defaults (font, how to display messages, etc) are all still there.
com.apple.mail.plist is a property list that stores your defaults (preferences) for the Mail application.

Deleting the file meant that Mail.app would on next launch use the factory defaults.

Why your instance of Mail was misbehaving, I can't tell you, but deleting the prefs sometimes helps in these cases.