homepage

I think the "premature" optimization comes down to that code:
"""
static int
put(Picklerobject *self, PyObject *ob)
{
 if (Py_REFCNT(ob) < 2 || self->fast)
 return 0;
 return put2(self, ob);
}
"""
(put2() being the function which emits BINPUT)
When you unpickle a dict, its contents are created anew and therefore the refcount is 1 => next pickling avoids emitting a BINPUT.