Comment by robert-brown

One big problem is that Python's semantics make it difficult to implement the language efficiently. When I translate the sieve code into safe Common Lisp, I get a speedup of 15. After adding a few declarations, making the code as unsafe as C, the speedup factor is 30. Those improvements are possible because the compiler can assume many things about the functions and operators used in the code. For instance, that the Lisp equivalents of range() and len() have not been redefined.