QSortFilterProxyModel is very slow with strings

[cimer84]

We have a problem with a simple QSortFilterProxyModel. My model is very slow when i use the sort method.
Can anybody fix my problem?
I put the code.

@flightsModel = new LroOptimalDelayedFlightPlansModel();
sortModel = new QSortFilterProxyModel(this);
sortModel->setSourceModel(flightsModel);
ui->tableViewFlightPlans->setModel(sortModel);
configurationViewFlightPlans();
connect(flightsModel, SIGNAL(sortModel()), this, SLOT(sortModelAfterValuesChanged()));

QVariant LroDelayedFlightPlansModel::data(const QModelIndex &index, int role) const
{
if((!index.isValid()) ||
(index.row() >= rowCount(index)))
{
return QVariant();
}

if(role == Qt::DisplayRole)
{

    if(index.column() == 0)
    {
        return QString::fromStdString(fp.getFPs(index.row()).getCallsign());
    }
    else if(index.column() == 1)
    {
        return QString::fromStdString(fp.getFPs(index.row()).getOpType());
    }
    else if(index.column() == 2)
    {
        if(fp.getFPs(index.row()).getOpType() == "DEP")
        {
            return QString::fromStdString(
                        fp.getFPs(index.row()).getDepartureTimes().getItot());
        }
        else
        {
            return QString::fromStdString(
                        fp.getFPs(index.row() ).getArrivalTimes().getIldt());
        }
    }
    else if(index.column() == 3)
    {
        if(fp.getFPs(index.row()).getOpType() == "DEP")
        {
            return QString::fromStdString(
                        fp.getFPs(index.row()).getDepartureTimes().getStot());
        }
        else
        {
            return QString::fromStdString(
                        fp.getFPs(index.row()).getArrivalTimes().getSldt());
        }
    }
    else if(index.column() == 4)
    {
        if(fp.getFPs(index.row()).getOpType() == "DEP")
        {
            return QString::fromStdString(
                        fp.getFPs(index.row()).getDepartureTimes().getFtot());
        }
        else
        {
            return QString::fromStdString(
                        fp.getFPs(index.row()).getArrivalTimes().getFldt());
        }
    }
    else if(index.column() == 5)
    {
        return QVariant(fp.getFPs(index.row()).getForecastDelay());
    }
    else if(index.column() == 6)
    {
        return QVariant(fp.getFPs(index.row()).getPunctualityDelay());
    }
    else if(index.column() == 7)
    {
        return QString::fromStdString(fp.getFPs(index.row()).getRunwayAllocation());
    }

    return QVariant();
}
else if(role == Qt::TextAlignmentRole)
{
    return Qt::AlignCenter;
}
return QVariant();

}
@