Can you help how to run these parallel?
Not possible in Qt. GUI objects (things inheriting from QWidget or QWindow) can be created only in the thread that owns QApplication, and you can have only a single QApplication instance.

You need to do an experiment, I think. Create a new QMainWindow-based application, put a QStackedWidget as the central widget for the main window, and then just add a bunch of QWidget-based pages to them. Don't use your existing classes. Use QTreeView, QTableView, QGraphicsView, whatever. Time how long that takes. I think you'll find that your bottleneck is not in building your GUI, it's whatever you are doing in your application's sub-window constructors that's the cause.