title: Why are Excel Array formulas slow?
body: <p>I have read in many places on the web that one of the downsides of using array formulas is that they are slow (and that using intermediate helper columns can speed things up).</p>
  
  <p>The general explanation given is that they are performing a lot of work. They are, but the helper formulas must surely also be reproducing the same amount of work spread across different cells. </p>
  
  <p>I would like to understand in a little more detail:</p>
  
  <ol>
  <li><strong>Why</strong> array formulas are slow? </li>
  <li>What situations might it not be so?</li>
  </ol>