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.