Modern browser javascript engines are way faster than CPython. My guess as to why: For over 10 years, one of the largest companies in the world (Google) has had a vested interest in improving the performance of the V8 javascript engine running in chrome. V8 is highly optimized for performance.

By comparison, there has never been such a concerted push to optimize pure python execution in CPython. Generally, if you need to get something done quickly in python, you just call into a library written in C or Fortran.