To reproduce:
private void radTreeView1_NodesNeeded(object sender, Telerik.WinControls.UI.NodesNeededEventArgs e)
{
    for (int i = 0; i < 100; i++)
    {
        var node = new RadTreeNode("Test"+i);
        for (int j = 0; j < 10; j++)
        {
            node.Nodes.Add(new RadTreeNode("Sub"));
        }
        node.Expanded = true;

        e.Nodes.Add(node);
    }
}

Workaorund:
private void radTreeView1_NodesNeeded(object sender, Telerik.WinControls.UI.NodesNeededEventArgs e)
{
    radTreeView1.BeginUpdate();
    for (int i = 0; i < 100; i++)
    {
        var node = new RadTreeNode("Test"+i);
        for (int j = 0; j < 10; j++)
        {
            node.Nodes.Add(new RadTreeNode("Sub"));
        }
        node.Expanded = true;

        e.Nodes.Add(node);
       

    }
    radTreeView1.EndUpdate();
}