dunno if it's 10.5.4 version of the OS specifically but i do know that mail on my macbook pro was super sluggish for the past 2 weeks - processor running really high, beachball, etc. as a test i uninstalled mail unread menu (removed the app, the pref file, mail bundle and another file - be sure to remove all 4) and sure enough, mail became very responsive again immediately.

it's a shame because this is really handy on a laptop where you're hiding the dock and can't see mail's dock notification icon.. oh well.