I'm currently working on a huge GUI made with Python and the delay in performance is not noticeable compared to the chore it would have been to program it on something like C/C++ (and I tried that already...).
If I had to say which language to use I would say:
-Python for prototyping and automation goodies. Amazing at scientific computing.
-C/C++ for high performance time consuming applications. Preferably as inner logic to Cython.
-Processing for eye candy and embedded apps. Preferably to beginner programmers.
-Javascript for HTML back and frontend.
https://impythonist.wordpress.com/2014/01/22/20-python-libraries-you-cant-live-without/