Async/Await with Task.Run UI Freezes without thread.sleep

Question

The below code works fine, does not lock the UI, but removing thread.sleep does. Just trying to understand why. Thanks

    public partial class MainFrm : Form
{
     public readonly SynchronizationContext UiContext = null;
     public MainFrm()
        {
        InitializeComponent();
        UiContext = SynchronizationContext.Current;
        }

    private async void Start_Click(object sender, EventArgs e)
    {
    
       await Task.Run(() =>
       {
               for (var i = 0; i <= 5000000; i++)
               {

                   Thread.Sleep(1); //anything but 0 works, having no sleep also fails, why?
                   UiContext.Post(o => { UItextBox.Text = $@"The integer is: {(int) o}"; }, i);
               }
       });
       
    }
}