清楚listbox绑定数据 ，然后再增加listbox数据绑定时会卡界面,该如何解决

清楚listbox绑定数据 ，然后再增加listbox数据绑定时会卡界面
贴上代码：
XAML部分：
 <Grid>
                <Grid.RowDefinitions>
                    <RowDefinition Height="*" />
                    <RowDefinition Height="2*" />
                 
                </Grid.RowDefinitions>

                <ListBox  x:Name="searchcontact" Grid.RowSpan="2" Margin="0,0,0,124"   >

                    <ListBox.ItemTemplate>
                        <DataTemplate>
                            <StackPanel>
                                <TextBlock Name="displayname" FontSize="35" Text="{Binding Displayname}"/>

                                <TextBlock Name="phonenum" Text="{Binding PhoneNum}"/>
                            </StackPanel>

                        </DataTemplate>
                    </ListBox.ItemTemplate>

                </ListBox>

                <Grid Grid.Row="1"  Margin="-12,0">
                    
                    <Button x:Name="buttonKeyPad1"  Margin="0,272,1,1" UseLayoutRounding="True" Click="buttonKeyPad_Click">
                        <StackPanel Orientation="Horizontal">
                            <TextBlock Margin="0,0,0,0" HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="50">
<Run Text="Add"/>
                            </TextBlock>
                            <TextBlock Margin="10,0,0,0" Foreground="Gray" HorizontalAlignment="Center" VerticalAlignment="Center" FontSize="15" MinWidth="25"/>
                        </StackPanel>
                    </Button>

                  
                </Grid>
            </Grid>

cs页代码：
 public class Person
    {
        public string Displayname { get; set; }
        public string PhoneNum { get; set; }
    }

    public partial class MainPage : PhoneApplicationPage
    {
        ObservableCollection<Person> plist = new ObservableCollection<Person>();
        // 构造函数
        public MainPage()
        {
            InitializeComponent();
            searchcontact.ItemsSource = plist;