> > I would like to know why that is the case.
> > If PyUnicode_Join is not optimal, then perhaps we should
> > better optimize it.
> 
> I don't know. _PyUnicodeWriter overallocates its buffer (+25%). It may
> reduce the number of realloc(), and so the number of times that the
> buffer is copied.

But PyUnicode_Join doesn't realloc() anything, since it creates a buffer
of exactly the right size. So this can't be the answer.