title: QListWidget performance with many custom items
body: <p>I have a list with about 2500 custom items. I set them with:</p>
  
  <pre><code> const std::vector&lt;const Items::AbstractItem *&gt; results = _engine.request(text);
      if (!results.empty())
      {
          for (auto i : results){
              QListWidgetItem *lwi = new QListWidgetItem;
              _results-&gt;addItem(lwi);
              ListItemWidget *w = new ListItemWidget;
              w-&gt;setName(i-&gt;name());
              w-&gt;setTooltip(i-&gt;path());
              _results-&gt;setItemWidget(lwi, w);
          }
          _results-&gt;setFixedHeight(std::min(5,_results-&gt;count()) * 48); // TODO
          _results-&gt;show();
      }
  </code></pre>
  
  <p>This takes about 5 seconds on an i5-4590. Hiding the widget is twice as fast. Is this normal or do I have look for errors?</p>