Where has this order of magnitude difference been "proved"? I would say that even developing in forth is not one order of magnitude slower than ruby, as long as reasonable libraries are available.
Yes, but the difference is not one order of magnitude. Not if there is some level of sanity in the language (e.g., I am not talking about brainfk). For example, even though one can program faster in Python, it is not 10 times faster than in C, especially for large-scale software.
I will however finish with, there's a reason C applications don't have features or agile development cycles, and it's not because the coders are absurdly productive. Also, please don't ever try to be a manager.