Microsoft reserves 20% of your available bandwidth for its own purposes (suspect for updates and interrogating your machine etc..) Here’s how to get it back:

Click Start–>Run–>type “gpedit.msc” without the “

This opens the group policy editor. Then go to:

Local Computer Policy–>Computer Configuration–>Administrative Templates–>Network–>QOS Packet Scheduler–>Limit Reservable Bandwidth

Double click on Limit Reservable bandwidth. It will say it is not configured, but the truth is under the ‘Explain’ tab :

“By default, the Packet Scheduler limits the system to 20 percent of the bandwidth of a connection, but you can use this setting to override the default.”

Source: Google Community