Robocopy that ships with Windows 7/2008 seems to have a bug. If you specify /MT (for multiple threads), it forces /E (copy empty directories). This causes it to run much more slowly. Even if all the destination directories have already been created, it still accesses each one. If copying over a relatively slow connection, it can be very slow if you use /MT. [more]
Some discussion is here: https://social.technet.microsoft.com/Forums/en-US/w7itprogeneral/thread/155d47ea-2523-4343-80dc-f0a987971b62