In [1]: i = 4

In [2]: timeit "Fizz" * (i % 3 == 0)
10000000 loops, best of 3: 35.6 ns per loop

In [3]: timeit "Fizz" if i % 3 == 0 else ""
10000000 loops, best of 3: 34.2 ns per loop

In [4]: i = 3

In [5]: timeit "Fizz" * (i % 3 == 0)
10000000 loops, best of 3: 35.8 ns per loop

In [6]: timeit "Fizz" if i % 3 == 0 else ""
10000000 loops, best of 3: 37.2 ns per loop