1) Use SystemInternal's process explorer and track all network connections: (As admin: Add column to track the network tx, rx counts)
https://docs.microsoft.com/en-us/sysinternals/downloads/proc...
2) When I see some app, system that connect to network which I think it should not. I just block their network connections with Windows Firewall or uninstall those app.
There is a "netsh advfirewall firewall" command that can do this from command line.
3) BTW this works very well with Windows' own service too. I block svchost.exe, Edge, SerchIndex, etc from accessing internet. svchost.exe is allowed to access local subnet for file, print sharing. When I feel like upgrade to to latest version windows 10. I turn off the firewall rules for a few hours to allow the update to go thru.
4) Only Firefox and Chrome in my windows system are allowed to connect to internet full time. The CPU usage is normally < 15% and most of the time < 10% even when playing youtube @ 1080 resolution.
The latest Firefox Quantum has less CPU usage and faster also. I love it.
5) The WSL (Windows Subsystem for Linux) is getting better - X, xfce4-termnial now working good enough for me. I like accessing the Ubuntu shell in windows environment without the overhead of Virtualbox.