Being a Python superset and being fast are fundamentally in tension. It would be possible, maybe, to have a Python superset where you get the highest performance as long as you avoided the dynamic features of Python. However, I suspect it would make the user base grumpy to have a hidden performance cliff that suddenly showed up when you used dynamic features (or depended on code that did the same).
The dynamic features of Python are no different from the dynamic features of Smalltalk, Self, Common Lisp, but people have been educated to expect otherwise due to the adoption failure of dynamic compilers in Python community.
