Yesterday around 3pm I noticed that my network connection become unresponsive. VPN was dropping every few minutes, browsing web was slow. Event speedtest.net was timing out.

I could not find anything wrong with my laptop. No malware, WiFi card was up at 144 mbit/sec. Rebooting network router didn't help either.

I started debugging and learn a new helpful command
from this post which is

netstat -aenos -p tcp  

Magic switch -o is there to show owning process ID.

Once I had PID it was just a matter of finding it in Process Explorer (or using tasklist command).

It turned out to be that Windows Update was downloading 2.1 GB of Service Pack 1 for Microsoft Office 2013 and other updates..