Update: It has been brought to my attention from a colleague that the idea of QOS taking up 20% of the bandwidth is actually a myth. Microsoft has an official response to this that you can read here and it`s been debunked on LifeHacker as well. Edit the setting at your own risk.
For their own use, Microsoft by default reserves 20% of your bandwidth for QOS or Quality of Service usage like Windows update. Well I don’t usually use Windows Update on a daily basis and I don`t think a majority of users do, so why reserve a bandwidth for it? To remove this limit, open up the Run interface then enter gpedit.msc:
This opens the group policy editor window. Under the Local Computer Policy choose Computer Configuration > Administrative Templates > Network > QOS Packet Scheduler > Limit Reservable Bandwidth. My screenshot below should guide you if you get lost on the navigation tree:
This will open the Limit reservable bandwidth properties window and you will see that by default it is not configured.
So why would you change a limit if it is not configured anyway? Well you can see the reason if you click 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.
Now that we know that limit is indeed 20% by default, go back to the Setting tab then choose Enabled and then put a zero value on the limit. Then click OK.
I am yet to benchmark the results of this setting. In theory it should add 20% to your bandwidth by removing this limit.