Thanks for the suggestion!

I have now implemented a mechanism which creates the MainView instances strictly sequential.

A background thread listens for events when to create main views. It then emits a signal to create the view, the GUI thread
listens for this signal and creates the view. While the GUI thread is creating the view, the thread waits until the GUI thread
notifies it via a signal that the main view has been created. Only then a new create request can be emitted.

This does actually not solve the problem that the creation of widgets takes much time, but all in all the GUI remains much
more responsive than creating all views at one time.

Thanks, best regards!