Extreme delay after selecting all items of a model with a large subtree and changing the sort order in ItemView (Headerview repaint)

bool QItemSelectionModel::isColumnSelected(int column, const QModelIndex &parent) const
{
. . .
    // add ranges and currentSelection and check through them all
    QList<QItemSelectionRange>::const_iterator it;
    QList<QItemSelectionRange> joined = d->ranges;
    if (d->currentSelection.count())
        joined += d->currentSelection;
    int rowCount = d->model->rowCount(parent);
    for (int row = 0; row < rowCount; ++row) {
         for (it = joined.constBegin(); it != joined.constEnd(); ++it) {
             if ((*it).contains(row, column, parent)) {
                 Qt::ItemFlags flags = d->model->index(row, column, parent).flags();
                 if ((flags & Qt::ItemIsSelectable) && (flags & Qt::ItemIsEnabled)) {
                     row = qMax(row, (*it).bottom());
                     break;
                 }
             }
         }
         if (it == joined.constEnd())
             return false;
    }
  return rowCount > 0; // no rows means no selected items
}

    for (int row = 0; row < rowCount; ++row) {
        Qt::ItemFlags flags = d->model->index(row, column, parent).flags();
        if ((flags & Qt::ItemIsSelectable) && !(flags & Qt::ItemIsEnabled)) {
            return false;
        }
    }