how to improve speed when we select control in GridView?

I have 100 checkbox in GridView ( 20x50)

I have 2 button upon GridView one is CheckAll another is UnCheckAll
when user click CheckAll , check all checkboxes
when user click UnCheckAll , uncheck all CheckBoxes


this is my code

    #region CheckAll
    protected void BtnAllAllow_Click(object sender, EventArgs e)
    {
        foreach (GridViewRow dr in GridView2.Rows)
        {
            for (int i = 4; i <= GridView2.Columns.Count - 1; i++)
            {
                if (dr.Cells[i].Controls[0] is CheckBox)
              &nb ...