You can create a status text in the App Designer by dragging "Label" onto your app in Design View. You can edit the font, alignment, and background color to fit your needs. You can uncheck the Visible check box. When calculation are being performed, you can add this to the callback that triggers the calculation:
set(app.Label, 'Visible', 'on'); drawnow;
Once it is done, you can add:
set(app.Label, 'Visible', 'off'); drawnow;
Note, you can rename your components by double clicking its name in the Component Browser. So you can change the name from app.Label to app.statusText, if you so desire.