问在Xamarin forms项目的iOS中搜索分组列表需要很长时间

EN

viewModel.FilterList(searchTicket.Text);

public ObservableCollection<Grouping<string, Ticket>> TicketList
{
    get { return _ticketList; }
    set { _ticketList = value; OnPropertyChanged(); }
}

public void FilterList(string text)
{
     GroupCollections(TicketListNew.Where(Tl => Tl.CustomerName.ToLower().Contains(searchText.ToLower());
}

 private void GroupCollections(List<Ticket> ticketListNew)
 {
     var sorted = from ticket in ticketListNew
                     orderby ticket.DelPUDate
                     group ticket by GetTicketDate(ticket.DelPUDate) into ticketListGroup
                     select new Grouping<string, Ticket>(ticketListGroup.Key, ticketListGroup);
      if (TicketList == null) TicketList = new ObservableCollection<Grouping<string, Ticket>>(sorted);
      else
      {
          TicketList.Clear();
          foreach (var item in sorted)
          {
              TicketList.Add(item);
           }
        }
    }