Can only think of something on the PC side maybe.
I would turn off everything, security SW, wired and wireless devices.
Use only one wired PC to the back of the router.
Ensure your FTP programs are set up and configured for speed or xfer as fast as possible. Might try other settings as well.
Might see if other end of FTP server is slowing you down.
I have been told that other DLink devices have seen this, DNS items.