One thing I haven't seen brought up in this thread yet is support for foreign language embeddability. For example, Python code is technically quite slow, but that often doesn't matter much because it is easy to write external functions in C or C++ that behave like normal Python functions. I imagine that it would be more difficult to embed C++ code into a language with strong functional gauruntees. In that sense, the performance of Python is close to the performance of the fastest language with the same paradigm, which is (with current compilers) better for imperative programs that are structurally more similar to how the CPU operates.
