High CPU load every few seconds

When starting Qt Creator and loading a project the CPU usage is still ok.
After switching once to the GUI designer the CPU load goes up every few seconds - without user interaction.

In Instruments it can be seen that there's a cycle that starts a new thread and does something related to network interfaces. Each thread consumes about 4 seconds of CPU time.

The application I built now with Qt 5.5.1 shows the same behavior which didn't occur with earlier versions (used 5.4 before).