title: Qt5 more slow than qt4 for draw many pixmaps in TabWidget
body: <p>I have a widget, and I add 300 pixmaps inside. </p>
  
  <pre><code>MyWidget::MyWidget( QWidget *parent )
  {
      setParent(parent);
      FlowLayout *flowLayout = new FlowLayout(this, 2,2,2);
      setLayout(flowLayout);
  
      QPixmap* cupcakes = new QPixmap("Resources/Icons/pimpCupcakes.png");
      TilePixmap* tilePximap = new TilePixmap(cupcakes, 0);
      m_buttonGroup = new QButtonGroup(this);
  
      int id = 0;
      for(int i=0; i&lt;300; ++i)
      {
          TilesetButton* buttonTmp = new TilesetButton(this, tilePximap);
          m_buttonGroup-&gt;addButton(buttonTmp, id);
          flowLayout-&gt;addWidget(buttonTmp);
          id++;
      }
  }
  </code></pre>
  
  <p>I had this widget in a QTabWidget like this : </p>
  
  <pre><code>ui.tabWidget-&gt;addTab(myWidget, name);
  </code></pre>
  
  <p>I do this operation with another widget</p>
  
  <pre><code>ui.tabWidget-&gt;addTab(myWidget2, name);
  </code></pre>
  
  <p>in qt4.8, creation (and first draw) need approximatively 4 secondes
  and after that, switch between tab 1 and 2 is instantaneous. </p>
  
  <p>in qt5, creation (and first draw) need approximatively 4 secondes (so like qt4.8)
  but after that, switch tab took always 4 secondes between the time when I clicked on the tab and when the signal "currentChanged(int)" emmited. </p>
  
  <p>Qt5 need add some cache configs compared with qt4 ?</p>
  
  <p>(project are identic, just the Qt version is different)</p>
  
  <p>Edit : 
  I do the same test not with pixmap but only with QPushButton* and a simple text on it.
  and it's the same problems.
  3/4 sec latency between my clic and the tab switch.</p>
  
  <p>I look if tabWidget code changed since qt4.8 but I saw nothing which could be the reason of this.</p>