title: Why could fetching resources over the network require multiple roundtrips between the client and server?
body: <p>I was reading at <a href="https://developers.google.com/speed/docs/insights/LeverageBrowserCaching" rel="nofollow noreferrer">https://developers.google.com/speed/docs/insights/LeverageBrowserCaching</a> the following information:</p>
  
  <blockquote>
    <p>Fetching resources over the network is both slow and expensive: the download may require multiple roundtrips between the client and server, which delays processing and may block rendering of page content, and also incurs data costs for the visitor.</p>
  </blockquote>
  
  <p>Why could this <code>require multiple roundtrips</code>? Why not <code>a single roundtrip</code>?</p>