c# WPF:添加项目占用时间

我试图将安全组从服务器添加到后台工作人员的组合框。但这需要时间，尽管有后台工作UI变得无响应。

 private void Accounts_Click(object sender, RoutedEventArgs e)
 {
if (servername.Text == "")
    MessageBox.Show("please enter server");
else
{
    mycombobox.Items.Clear();
    server1 = servername.Text;
    bgw_account = new BackgroundWorker();
    bgw_account.WorkerReportsProgress = true;
    bgw_account.ProgressChanged += new ProgressChangedEventHandler(bgw_account_ProgressChanged);

    bgw_account.RunWorkerCompleted += new RunWorkerCompletedEventHandler(bgw_account_RunWorkerCompleted);
    bgw_account.DoWork += new DoWorkEventHandler(bgw_account_DoWork);
    bgw_account.RunWorkerAsync();
  }
}

      private void bgw_account_DoWork(object sender, DoWorkEventArgs e)
    {
      ParallelOptions po = new ParallelOptions();
      po.MaxDegreeOfParallelism = System.Environment.ProcessorCount;
      acclist = new List<string>();
      using(PrincipalContext pt= new PrincipalContext(ContextType.Machine,server1.ToString()))
      using (GroupPrincipal ab = new GroupPrincipal(pt))
      using (var searcher=new PrincipalSearcher(ab))
      using (var results=searcher.FindAll())
       {
        Parallel.ForEach(results,po,(currentsam)=>
        {
        acclist.Add(currentsam.SamAccountName.ToString());//timeconsuming task
        });

       }

  }
       private void bgw_account_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
      {
        mycombobox.Dispatcher.Invoke(new Action(() =>
       {
         mycombobox.Items.Clear();
        }));
           foreach (string p in acclist)
          {
           mycombobox.Dispatcher.Invoke( new Action (()=>
            {
             mycombobox.Items.Add(p);
             }));
          }

        acclist.Clear();
      }     

有办法提高性能吗?