使用backgroundWorker组件，为啥还会占用主线程，导致卡的现象

使用backgroundWorker组件，为什么还会占用主线程，导致卡的现象?
直接贴代码
打断点看的时候，主线程执行完backgroundWorker1.RunWorkerAsync();后 断点出了这里-> }  就一直卡着界面，要等OldUpload运行完才有反应

        private void btnOldUpload_Click(object sender, EventArgs e)

        {

            backgroundWorker1.RunWorkerAsync();

        }   

        private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)

        {

             OldUpload();

        }


        private void backgroundWorker1_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)

        {

           if (e.Error != null)

                Console.WriteLine("error");

            else

            {

                MSG("上传结束!");

            }       

        }


       /**需要长时间执行的方法**/

        private void OldUpload()

        {

            int ucount = dgvUsers.Rows.Count;

            int mcount = dgvOldMachines.Rows.Count;

            for (int i = 0; i < mcount; i++)//

            {

                dgvOldMachines.EndEdit();


                DataGridViewCheckBoxCell ckoldmachine = (DataGridViewCheckBoxCell)dgvOldMachines.Rows[i].Cells["xuanold"];

                Boolean flag = Convert.ToBoolean(ckoldmachine.Value);

                if (flag == true)//datagridview 设备选中的信息                {

                    int iMachineNumber = Convert.ToInt32(dgvOldMachines.Rows[i].Cells[2].Value.ToString());

                    MachineInfo m = MachineInfoBLL.SelectMachineInfoByEqpID(iMachineNumber.ToString());


                    for (int j = 0; j < ucount; j++)

                    {

                        dgvUsers.EndEdit();

                        DataGridViewCheckBoxCell checkcell = (DataGridViewCheckBoxCell)dgvUsers.Rows[j].Cells["Column1"];

                        Boolean flagm = Convert.ToBoolean(checkcell.Value);


                        if (flagm == true)//datagridview人员选中的信息

                        {

                            try

                            {

                                int idwEnrollNumber = Convert.ToInt32(dgvUsers.Rows[j].Cells[1].Value.ToString());

                                string sName = dgvUsers.Rows[j].Cells[2].Value.ToString();

                                if (Connect(axCZKEM1, m.Ip, m.Port, iMachineNumber.ToString(), Convert.ToInt32(m.Commpassword)))