OK, thanks. FYI and OT - Here is what i found: more strange behavior 
I logged in via wifi and changed the user to root. Had a very short look on top - haven’t seen nothing suspicious and installed powertop additionally. Then I just wanted to connect the phone to the cable and the pop up menu for media transfer vs. just charging vs. developer flickered 3 or 4 times after i was able to select just charging mode. I checked top and powertop and wanted to compare for a first try what is happening when I disconnect the phone. Again, nothing what directly seems the reason for a high load. And here is whats new. After I disconnected the phone I realized that the CPU utilization stays low. So I think this flickering caused something.
I know there is a bug with voicecall ui and I realized, that this high usage on my phone somehow correlates everytime I miss a call. But this charging thing is new. 