Unresponsiveness with async event handlers in WPF in .NET 4.5

I have created a simple async operation which is being kicked of when the button is clicked. Here is the whole code:

public partial class MainWindow : Window {

    public MainWindow() {
        InitializeComponent();
    }

    private async void Button_Click_1(object sender, RoutedEventArgs e) {

        var htmlString = await DowloadPage("http://example.com");
        txtBlock1.Text = htmlString;
    }

    public async Task<string> DowloadPage(string uri) {

        using (WebClient client = new WebClient()) {

            var htmlString = await client.DownloadStringTaskAsync(uri);
            return htmlString;   
        }
    }

}

Very easy. But when I click the button, I experience unresponsiveness on the UI thread. When I try to move around the window while the page is being downloaded, I am unable to.

Any idea what is going wrong?

Edit:

I tried with HttpClient in .NET 4.5 and it worked out pretty great as expected:

public async Task<string> DowloadPage(string uri) {

    using (HttpClient client = new HttpClient()) {

        var response = await client.GetAsync(uri);
        var htmlString = await response.Content.ReadAsStringAsync();
        return htmlString;   
    }
}

• BTW there is a new HttpClient Class in .NET 4.5 which is designed for async/await. - dtb
• @dtb I am aware of that. Thanks but the issue here is different. - tugberk
• How long is htmlString? It may simply take some time to render the txtBlock1 with it. - dtb
• @dtb do you think that newing up the WebClient would cause that blocking? - tugberk
• No, creating a WebClient instance is really lightweight. Have you checked how long htmlString is (see my comment above)? - dtb