I've noticed that every time I run a Mail applescript there is a 3-5 second delay before something actually happens in Mail. The next time I run the script everything happens at once, then if I wait for awhile I'm back to the delay. This happens on Snow Leopard and Leopard.
I think this is Mail thing but if you have any hint/tips
tell application "Mail" to activate
tell application "System Events"
tell process "Mail"
tell menu bar 1
click menu bar item "Message"'s menu "Message"'s ¬
menu item "Reply"
key code 117 -- Forward Delete
key code 125 using command down -- down arrow
key code 36 -- Return