System so slow; permanent WiFi activity I guess my Android device is permanent communicating over the Internet, without any doing of my side. This slows down the system.

Is there any way to see the actual WiFi activity? I mean I like to know which app / service is actually downloading or sending out information. I tested several task-managers with WiFi Monitoring but all do summarize. I like to know which app at this moment downloads or sends data without my doing.
Best would be a widget that shows which app uses WiFi at most.
OS Monitor might prove helpful here. Amongst others, it shows you all current connections including their stati, always along with the target IP (which you can even get WhoIs details on, see screenshots) and the app holding it. This might at least help you to narrow down possible candidates.

 
OS Monitor: Open connection, connection target details (source: Google Play; click images for larger variants)

Hint: To transfer data, the connection status must be ESTABLISHED.