title: difference of two arrays with 10 million items - _.difference is too slow
body: <p>I have two arrays with user id's and I want to check different items in them.</p>
  
  <pre><code>arr1 = [123, 456, 789];
  arr2 = [123, 456, 789, 098];
  </code></pre>
  
  <p>The problem is: these arrays can have 10 or 20 millions of items.</p>
  
  <p>I'm trying with <code>underscore.difference()</code> but it took 10 mins to complete.</p>
  
  <p>Is there any faster way to do this?</p>