Instagram also uses Cython a lot (so I've heard from a talk), so switching away from CPython might not provide as much of a speed up as one might otherwise expe

Instagram also uses Cython a lot (so I've heard from a talk), so switching away from CPython might not provide as much of a speed up as one might otherwise expect, as one can get C levels of speed (and concurrency) with significant effort using Cython. Also, Cython and PyPy might not play so well together...