How to improve the performance while selecting row using shift key in WPF DataGrid (SfDataGrid)?

public class SfDataGridBehavior : Behavior<SfDataGrid>
{
    SfDataGrid datagrid = null;
    protected override void OnAttached()
    {
        datagrid = this.AssociatedObject as SfDataGrid;
        datagrid.SelectionController = new GridSelectionControllerExt(datagrid);
        base.OnAttached();
    }
}

public class GridSelectionControllerExt : GridSelectionController
{
    SfDataGrid datagrid;
    public GridSelectionControllerExt(SfDataGrid dataGrid): base(dataGrid)
    {
        datagrid = dataGrid;
    }
}

   /// <summary>
    /// Processes the row selection when the mouse pointer is released from SfDataGrid. 
    /// </summary>
    /// <param name="args">
    /// Contains the data for mouse pointer action.
    /// </param>
    /// <param name="rowColumnIndex">
    /// The corresponding rowColumnIndex of the mouse released point.
    /// </param>
    /// <remarks>
    /// The selection is initialized in pointer released state when the <see cref="Syncfusion.UI.Xaml.Grid.SfDataGrid.AllowSelectionPointerPressed"/> set as false.        
    /// </remarks>
    protected override void ProcessPointerReleased(MouseButtonEventArgs args, Syncfusion.UI.Xaml.ScrollAxis.RowColumnIndex rowColumnIndex)
    {
        if (SelectionHelper.CheckShiftKeyPressed() && this.PressedRowColumnIndex.RowIndex > 0)
        {
            this.SelectRows(this.PressedRowColumnIndex.RowIndex, rowColumnIndex.RowIndex);
        }
        else
            base.ProcessPointerReleased(args, rowColumnIndex);
    }
 
    /// <summary>
    /// Selects the rows corresponding to the specified start and end index of the row.
    /// </summary>
    /// <param name="startRowIndex">
    /// The start index of the row.
    /// </param>
    /// <param name="endRowIndex">
    /// The end index of the row.
    /// </param>
    public override void SelectRows(int startRowIndex, int endRowIndex)
    {
        if (startRowIndex < 0 || endRowIndex < 0)
            return;
 
        if (startRowIndex > endRowIndex)
        {
            var temp = startRowIndex;
            startRowIndex = endRowIndex;
            endRowIndex = temp;
        }
 
        if (this.DataGrid.SelectionMode == GridSelectionMode.None ||
            this.DataGrid.SelectionMode == GridSelectionMode.Single)
            return;
 
        var isSelectedRowsContains = this.SelectedRows.Any();
 
        this.SuspendUpdates();
        var addedItem = new List<object>();
        int rowIndex = startRowIndex;
 
        this.datagrid.SelectedItems.Clear();
        var rowIndexes = this.datagrid.SelectionController.SelectedRows.Select(rowinfo => rowinfo.RowIndex).ToList(); 
 
        var selectedrowindex = this.datagrid.ResolveToRowIndex(this.datagrid.SelectedIndex);
 
 
        for (int i = rowIndex; i <= endRowIndex; i++)
        {
            object rowData = this.DataGrid.GetRecordAtRowIndex(i);
 
            if (!rowIndexes.Contains(i))
            {
                this.SelectedRows.Add(GetGridSelectedRow(i));
                this.DataGrid.ShowRowSelection(i);
            }
            if (rowData != null)
            {
                this.DataGrid.SelectedItems.Add(rowData);
            }
        }
 
        if (!isSelectedRowsContains)
        {
            this.DataGrid.SelectedIndex = this.SelectedRows.Count > 0 ? this.DataGrid.ResolveToRecordIndex(this.SelectedRows[0].RowIndex) : -1;
            this.DataGrid.SelectedItem = this.DataGrid.SelectedItems.Count > 0 ? this.DataGrid.SelectedItems[0] : null;
 
        }
        this.ResumeUpdates();
    }
}