title: Initial DT Processing has a slow network request
body: <p>the "Processing..." step of a <a href="https://github.com/rstudio/DT" rel="nofollow noreferrer">DT</a> in my Shiny App takes a long time as soon as the app does not run locally on my PC, but on a shiny server.
  I profiled the app using profvis (remotely on the server), but the " DT::renderDataTable" step takes only ~400ms (approx. 2000 rows).</p>
  
  <p>Chrome however shows a "Network request step" that takes ~15secs:</p>
  
  <p><img src="https://i.stack.imgur.com/XfBCB.png" alt="Chrome Profiling Screenshot"></p>
  
  <p>Info from the Chrome performance analysis:</p>
  
  <ul>
  <li>Network request URL
  /session/d229229…/dataobj/dataTableOut?w=&amp;nonce=ce39... </li>
  <li>Duration 15.89 s</li>
  <li>Request Method POST</li>
  <li>Priority High</li>
  <li>Mime Type application/json</li>
  <li>Encoded Data 1.7 KB</li>
  <li>Decoded Body 5.2 KB</li>
  <li>Initiator jquery.min.js:2</li>
  </ul>
  
  <p>What is causing this delay?</p>
  
  <p>Thanks for your help!</p>