title: the fastest way to compute adjacent edge list (as_adj_edge_list)
body: <p>This problem almost drives me crazy. First say I have a sample <strong>directed</strong> graph like:</p>
  
  <pre><code>g &lt;- sample_gnp(20000, 0.1, directed = T)
  </code></pre>
  
  <p>Now I want to compute the proportion of "mutual edges" for one's all outgoing edges. This is the fastest way that comes to my mind:</p>
  
  <pre><code>ael &lt;- as_adj_edge_list(graph, mode = "out")
  sapply(ael, function(x) {mean(which_mutual(g, x))})
  </code></pre>
  
  <p>However, the <code>as_adj_edge_list</code> function seems to take endless time. My real data has about 23k vertices and 110k edges so I cannot just set and wait for the code to finish. Besides, from a search of SO posts, I find that <code>as_adj_edge_list</code> is already the fastest way to iterate over vertices or edges. So am I using it the stupid way? Many thanks in advance. </p>