[foldr]

> pypy, which is 50 times faster than V8.

Ehhh V8 is pretty competitive with C for number crunching. Where are you getting that 50 times figure from?

[KaiserPro]

threads.

[foldr]

http://doc.pypy.org/en/latest/faq.html#does-pypy-have-a-gil-...

If you're using parallelism for non-IO-related reasons, then in either language, you'll probably end up using multiple processes rather than multiple threads.