Just a slight note - Python is not as slow as you might think, but it requires more 'planning' on how you do stuff if you run into speed limits. Also, what has not been mentioned here, it's fairly easy to mix C++ and Python, so you can replace the more intensive functions with C++ code, minimizing speed differences.

PS. and to correct the statement from a few posts above - PyQt apps require the same amount of root space as regular Qt apps do, no disadvantage there.