Microsoft reserves 20% of your available bandwidth for their own purposes like Windows Updates and interrogating your PC etc

You can get it back:

1.      Go to run (Win+R)
2.      Type gpedit.msc in the box
3.      Now a window will appear where u will be seeing computer configuration and in that you will see Administrative Templates.
4.      Click once on Administrative Templates.
5.      On the right side you will be seeing Network , double click on Network tab.
6.      Then you will see QoS Pocket Schedular. double click on QoS Pocket Schedular.
7.      Apter then double click on Limit Reservable Bandwith.
8.      Now another window will appear, and now you will be seeing tick mark on Not Configured, now just unmark that and click on Enabled.
9.      Now below you will be seeing Bandwith Limit(%)  and in box there you will see 20%. You just need to replace that 20 to 0 .
10.  Now click on OK and Then click on Apply.
11.  You are done, restart your computer and start surfing internet.