QtCreator freezes system when starting and running GUI Applications

When starting QtCreator 4.12.0 (clicking a dock icon) the system freezes for about 10 seconds. After QtCreator started the system freezes again for about 10 seconds.

Starting an empty Widget-based application freezes the system for about 10 seconds after compilation.

Starting a QML-based application freezes the system for about 30 seconds after compilation.

Console applications seem not to be affected.

When the system is not responsive i can not switch applications or move any window, i can move the mouse though. Only after the application successfully started (window is visible) the system is responsive again.

Starting GUI applications from the Finder with a double-click works fine as expected.