Speed up moving mails in Mail with AppleScript

Moving or deleting mails in Mail with AppleScript is dog slow. One of my scripts is:

tell application "Mail" set SelectedMails to selection

repeat with currentMail in SelectedMails
    set isRead to get read status of currentMail
    if isRead then
        move currentMail to mailbox "xxx" of mailbox "yyy"
    end if
end repeat

end tell

The speed per mail is longer than a second. Deleting mails is similar in lack of speed. Does anyone have an idea how to speed this up?