I only just noticed you replied again.
Looking at the processes in the task manager, I see that it is a process called "desktopfenster manager", which causes the high cpu load.
WH window minimized - 0,7 - 1,7%
WH window maximizied, but in the background - 2,2 - 3,6%
WH window partly covered, turning arrow visible - 16,6 - 17,9%
WH window partly covered, turning arrow covered - 4,5%
pomodoro window partly covered - 4,5 - 5,2%
Something with the display of counting numbers and turning arrows seems to be very inefficient somewhere, but I am no programmer, so...
Hope that helps to clarify the issue.