title: Calculating mean of a dataframe takes way longer than the standard deviation
body: <p>I have a very large pandas Dataframe for which I am trying to find the std and mean. The It takes only few seconds to calculate the std while finding the mean takes in the order of minutes. Am I missing something or we have to calculate the mean first to calculate the standard deviation? How come calculating the mean takes much more time?</p>
  <pre><code>df = pd.read_csv('large_df.csv')
  std = df.std()
  mean = df.mean()
  </code></pre>