timeit('from __main__ import np, pd, a, b; pd.Series(np.concatenate([a,b]))',
    number=10000)
timeit(
    'from __main__ import np, pd, a, b; pd.concat([a, b], ignore_index=True)',
    number=10000)
timeit('from __main__ import np, pd, a, b; a.append(b).reset_index(drop=True)',
    number=10000)
Simple way to append a pandas series with same index