My Program Is Being Slow On CellFormating Event How Fix That?

Question

I Have A Function In My CellFormating Event And That Function Have Two For Loop To Check That In A Row Exist A NULL Cell Or Not And Then Change The Row Header Of DataGridView

But My Program is Slow Down And I Cant Do Any Thing Any More How Can I Fix That ??

    private void dgv_tests_CellFormatting(object sender, DataGridViewCellFormattingEventArgs e)
            {
                RowHaveNull();
            }

 private void RowHaveNull()
        {
            for (int i = 0; i < dgv_tests.Rows.Count; i++)
            {
                for (int j = 0; j < dgv_tests.Columns.Count; j++)
                {
                    dgv_tests.Rows[i].HeaderCell.Style.BackColor = MyGreen;

                    if (dgv_tests.Rows[i].Cells[j].Value == null || dgv_tests.Rows[i].Cells[j].Value.ToString() == "")
                    {
                        dgv_tests.Rows[i].HeaderCell.Style.BackColor = MyYellow;
                        break;
                    }
                }
            }
        }