title: pandas fillna(method='ffill') vs ffill()
body: <p>Hi My dearly pandas gurus,</p>
  <p>I was trying to forward fill missing data in a dataframe, these were the two ways I compared, the ffill() outperformed fillna(method='ffill') tremendously, so here are questions popping up, are they doing the same thing to the dataframe, if they are same, why are they so different in performance. is ffill() implemented in cython?</p>
  <pre><code>df = pd.DataFrame([[np.nan, 2, np.nan, 0],
                  [3, 4, np.nan, 1],
                  [np.nan, np.nan, np.nan, 5],
                  [np.nan, 3, np.nan, 4]],
                 columns=list('ABCD'))
  df.fillna(method='ffill')
  #vs
  df.ffill()
  </code></pre>