Thanks for reply.

1. I create a instance of XamWebGrid and set a event handler to KeyDown event.
2. set Data (it has 10,000 rows) to XamWebGrid.ItemsSource.
3. select all rows by XamWebGrid.SelectionSettings.SelectedRows.Add

Process takes about 20 sec at step 3.

But, User can select all rows by mouse, "Click first row and Shift-Click last row".
It takes about 0 sec.

So, I say "too slow".

I think SelectedRows.Add is too slow.

Code is this:

using System.Collections.Generic;
using System.Windows;
using System.Windows.Controls;
using Infragistics.Silverlight.Controls;
using Infragistics.Silverlight;

namespace sample
{
    public partial class MainPage : UserControl
    {
        private XamWebGrid webGrid;

        public MainPage()
        {
            InitializeComponent();
        }

        private void ButtonSearch_Click(object sender, RoutedEventArgs e)
        {
            webGrid = new XamWebGrid();
            webGrid.EditingSettings.AllowEditing = EditingType.None;
            webGrid.SelectionSettings.CellClickAction = CellSelectionAction.SelectRow;
            webGrid.SelectionSettings.RowSelection = SelectionType.Multiple;

            webGrid.KeyDown
                += new System.Windows.Input.KeyEventHandler(onKeyDown);

            Grid.SetColumn(webGrid, 1);
            LayoutRoot.Children.Add(webGrid);

            List<sampleWebService.ITEM> testdata = new List<sampleWebService.ITEM>();
            for (int i = 0; i < 10000; ++i)
            {
                sampleWebService.ITEM item = new sampleWebService.ITEM();
                item.ATTR1 = i.ToString();
                testdata.Add(item);
            }
            webGrid.ItemsSource = testdata;
        }

        private void onKeyDown(object sender, System.Windows.Input.KeyEventArgs e)
        {
            // Ctrl-A
            if (e.Key == System.Windows.Input.Key.A
                && System.Windows.Input.Keyboard.Modifiers == System.Windows.Input.ModifierKeys.Control)
            {
                foreach (var x in webGrid.Rows)
                {
                    webGrid.SelectionSettings.SelectedRows.Add(x);
                }
            }
        }
    }
}