Append Model to VXYModelMapper

Hi,
I use "VXYModelMapper" and "QStandardItemModel" in plot chart. I use following code but append model to VXYModelMapper is very slow. QElapsedTimer shows the delay in the last line.
I consume 123045 milliseconds if I do it after showing the window, if I do it before it consumes 0 millisecond :O. Well, I need the user to select a chart from the list to display it. So the code runs after after showing the window.
version of Qt: Qt 5.11.2

Code:

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VXYModelMapper {
        model: myChartClass.newMyChartModel
        series: lineSeries
        xColumn: 0
        yColumn: 1
}
void MyChartClass::setMyChartModel(QStandardItemModel *model)
{
  newMyChartModel= model;
  emit myChartModelChanged(model);
}
 
QStandardItemModel* lineModel=new QStandardItemModel(npcArray.size(), 2);
foreach(const QJsonValue & val, npcArray){
    double xVal=val.toObject().value("x").toDouble();
    double yVal=val.toObject().value("y").toDouble();
 
    QStandardItem *item1 = new QStandardItem(QString::number(xVal));
    lineModel->setItem(i, 0, item1);
 
    QStandardItem *item2 = new QStandardItem(QString::number(yVal));
    lineModel->setItem(i, 1, item2);
     i+=1;
}
 
 //To this line everything is very good and fast   
 
QElapsedTimer timer;
timer.start();
myChartClass->setMyChartModel(lineModel);
qDebug() << "The slow operation took" << timer.elapsed() << "millisecond";

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Output:

Code:

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The slow operation took 123045 millisecond. 
Total point count: 5188

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It is very slow :(
Please guide me.