title: Gaps in FireBug waterfall chart
body: <p>When testing performance of various web pages on my web application I noticed that there are some gaps in net tab (waterfall chart) in firebug.  In some cases these may take up half of the time for the whole request.</p>
  
  <p>What usually causes these gaps and how can they be removed?</p>
  
  <p><img src="https://i.stack.imgur.com/1BDAR.png" alt="enter image description here"></p>