Append to XYDataSeries is slow for the first time

Hi

I am creating a simple scatter chart, below is the code in ViewModel for getting data series, mostly from sample code.
My scenario is to be able to add this chart on a button click, so it can add multiple of this chart.
Issue is first time I click the button, dataSeries.Append works very slow, like it takes 10 seconds to pass that line, my laptop has very fast cpu and a lot of ram, so it is not hardware problem. After I added first chart, all subsequent charts added very fast, including different type of charts.
Could you please advise? thanks.

    private XyDataSeries<double, double> GenerateDataSeries(double startsAt, double[] data)
    {
        var dataSeries = new XyDataSeries<double, double>();
        var xValues = GeneratexValues(startsAt, data.Length);
        dataSeries.Append(xValues, data); <=====================
        return dataSeries;
    }