UI线程在文本框调用期间冻结

为什么在从分离线程调用文本框时UI冻结

    private void button1_Click(object sender, EventArgs e)
    {
        Thread t1 = new Thread(DoStuff);
        t1.Start();
    }
    void DoStuff()
    {
        using (var wc = new System.Net.WebClient())
        {
            string page_src = wc.DownloadString("http://bing.com");
            textBox1.Invoke((MethodInvoker)delegate() { textBox1.Text = page_src; }); // freezes while textbox text is changing
        }
    }

同时backgroundworker工作完美- UI不会冻结

    private void button1_Click(object sender, EventArgs e)
    {
        BackgroundWorker bw1 = new BackgroundWorker();
        bw1.DoWork += (a, b) => { DoStuff(); };
        bw1.RunWorkerAsync();
    }
    void DoStuff()
    {
        using (var wc = new System.Net.WebClient())
        {
            string res = wc.DownloadString("http://bing.com");
            textBox1.Invoke((MethodInvoker)delegate() { textBox1.Text = res; }); // works great
        }
    }