title: Qt Sort using lessThan is extremely slow
body: <p>I have an QTableView with QSortFilterProxyModel (on QFileSystemModel). I sort it using lessThen comparator like this:</p>
  
  <pre><code>bool   SortProxy::lessThan ( const QModelIndex &amp; left, const QModelIndex &amp; right ) const
   {
  ...
  return (collator.compare(left.data().toString(),right.data().toString())) &gt; 0;
  ... 
  }
  </code></pre>
  
  <p>And it works fine for sorting files by name, or filetype. But it works extremely slow on sorting files by name. What to do? </p>