def _adjust_returns(returns, adjustment_factor):
    """
    Returns the returns series adjusted by adjustment_factor. Optimizes for the
    case of adjustment_factor being 0 by returning returns itself, not a copy!

    Parameters
    ----------
    returns : pd.Series or np.ndarray
    adjustment_factor : pd.Series or np.ndarray or float or int

    Returns
    -------
    pd.Series or np.ndarray
    """
    if isinstance(adjustment_factor, (float, int)) and adjustment_factor == 0:
        return returns
    return returns - adjustment_factor