private void button3_Click(object sender, EventArgs e)
        {
            listView1.Groups.Clear();
            listView1.Items.Clear();
            for(int i =0;i<2000;i++)
            {
                Random rnd = new Random();
                int k = rnd.Next(1, 13);
                listView1.Groups.Add(new ListViewGroup("zzz "));
                for(int j=0;j<k+1;j++)
                {
                    ListViewItem item = new ListViewItem();
                    item.Text = "zzz";
                    item.SubItems.Add(rnd.Next(1, 13).ToString());
                    item.Group = listView1.Groups[listView1.Groups.Count - 1];
                    listView1.Items.Add(item);
                }
            }
        }

        private void button4_Click(object sender, EventArgs e)
        {
            for(int i=0;i<listView1.Groups.Count;i++)
            {
                int z = 0;
                for (int j = 0; j < listView1.Groups[i].Items.Count; j++)
                    z += Convert.ToInt32(listView1.Groups[i].Items[j].SubItems[1].Text);
                listView1.Groups[i].Header += z.ToString();//тут тупит
            }
        }