“Robocopy's “inter-packet gap” (IPG) option allows some control over the network bandwidth utilized in a session. In theory, the following formula expresses the delay (D, in milliseconds) required to simulate a desired bandwidth (BD, in kilobits per second), over a network link with an available bandwidth of BA kbps:”
IPG in ms = ((B_A - B_D)/(B_A * B_D)) * 512 * 1000
B_A = Bandwidth Available
B_D = Bandwidth Desired
Source:filepie.us - robocopy