Yet one case for comparison (with msg270095):

$ ./python -m timeit -s "x = 2" -- 'f"X is {x!s}"'
1000000 loops, best of 3: 0.625 usec per loop

Seems f'{x!s}' is the fastest way to stringify a value. Thus there is an opportunity to speed up default formatting by special casing PyObject_Format() for most popular types (str and int).