Adding a Network Monitor widget to the desktop makes plasmashell CPU's usage steadily increase and it shortly reaches 100 %.
Steps to Reproduce:
1. Add a Network Monitor widget to the desktop
2. Do something that include network usage, like web browsing, etc...
3. Monitor the CPU usage
CPU usage of plasmashell steadily increases to 100%
Plasmashell CPU usage remains low, or at least stay at constant level.
*** This bug has been marked as a duplicate of bug 345696 ***
Just to confirm thay I had the same problem with Plasma Shell hitting 138%. Removing the widget solved the highload.