I think you're both correct... the big question here is whether there is a good library to do the heavy lifting in your app. If there is, then choice of library will matter more than the app language, as long as there are bindings for your language of choice.
IF, and this is a big IF, you're doing something weird enough that you have to do the heavy lifting yourself, choosing an efficient language will matter a lot. At least for the heavy-lifting parts - you could still implement most of the the app in an easy inefficient language, and only do the heavy parts in C.
That's what happens in Python, and that's why my Python apps are only 3-10 times slower than pure C ("only" considering that Python is likes 100x slower than C).
IF, and this is a big IF, you're doing something weird enough that you have to do the heavy lifting yourself, choosing an efficient language will matter a lot. At least for the heavy-lifting parts - you could still implement most of the the app in an easy inefficient language, and only do the heavy parts in C.
That's what happens in Python, and that's why my Python apps are only 3-10 times slower than pure C ("only" considering that Python is likes 100x slower than C).