Changing Tkinter Label Text Dynamically using Label.configure()
After you change the text to "Process Started", use label.update(). That will update the text before sleeping for 5 seconds.
Tkinter does everything in its mainloop, including redrawing the text on the label. In your callback, it's not able to draw it because your callback hasn't returned yet. Calling update tells tkinter to run all the tasks it needs to on the label, even though your code is still running.