Use /IPG:n option to reduce robocopy’s impact on a slow or busy network link.
Between each 64Kb packet, robocopy will wait for n milliseconds so other applications can use the whole bandwidth.
I found an interesting link on zeda.n to calculate the transfert time when IPG delay (the n variable):
For instance to mirror a local directory tree to a remote location with a 10ms delay :
robocopy L:\FILES\SHARE\buro \\mynas1\SHARE12\buro /copyall /mir /r:5 /w:10 /log:c:\robo_buro.log /np /IPG:10