I think he might have meant 2x slower than without STM and not 2x slower than CP...

Niedar on Aug 23, 2011 | parent | context | favorite | on: PyPy - We need Software Transactional Memory

I think he might have meant 2x slower than without STM and not 2x slower than CPython. Also from everything I have read you would still have the option to not use STM at all and just use the GIL which would not come with the extra overhead.