This policy setting limits the network bandwidth that BITS uses for peer cache transfers (this setting does not affect transfers from the origin server). To prevent any negative impact to a computer caused by serving other peers by default BITS will use up to 30 percent of the bandwidth of the slowest active network interface. For example if a computer has both a 100 Mbps network card and a 56 Kbps modem and both are active BITS will use a maximum of 30 percent of 56 Kbps. You can change the default behavior of BITS and specify a fixed maximum bandwidth that BITS will use for peer caching. If you enable this policy setting you can enter a value in bits per second (bps) between 1048576 and 4294967200 to use as the maximum network bandwidth used for peer caching. If you disable this policy setting or do not configure it the default value of 30 percent of the slowest active network interface will be used. Note: This setting has no effect if the "Allow BITS peer caching" policy setting is disabled or not configured.
Network\Background Intelligent Transfer Service (BITS)
At least Windows Vista