Did you know that it’s possible to specify a bandwidth limit when copying with robocopy? To set a bandwidth limit in robocopy, use the /IPG parameter. For example:
robocopy c:\source d:\destination /IPG:1
The above robocopy example copies the files in the source to the destination with a 1ms pause between each packet.
This clever chap has created a formula to work out how to calculate the number of ms you need to enter, given your links current bandwidth, to copy a file at a specific bandwidth speed.