How to find a performance hog app in plasma?

[Stakanov]

Hello everyone. 

I am running opensuse Tumbleweed with KDE. 

Recently I experience a Problem that is limited to one user. The Plasma 
desktop causes a close to 100 % load on one CPU all dedicated to the plasma 
desktop (top). 

Now I have another user on the machine: 5-10% load systemwide when running. 

As the settings for rendering, etc are practically identical, the issue does 
not seem to belong to plasma, but (probably) to an app running in plasma?
How can i debug which app in plasma uses how much CPU power to understand the 
reason for this problem? 

Any suggestion for a strategy for advancing efficiently as well as effectively 
in debugging this? 

Any known issue that may have arisen recently and a known workaround / fix? 

Is there a way to list selectively all active app instances in plasma?

Thank you in advance for any help.