您的方法是同步执行的，因为它们都没有返回实际的Task。但最重要的是，您未能达到目标，因为您没有卸载CPU密集型工作以减轻UI线程的负担。正是这个循环仍然耗尽UI线程，因此冻结了UI。
要将其移动到不同的线程（后台线程），必须调用Task.Run：

private async void _btnStartClick(object sender, EventArgs e)
{
  // Call to Method which no longer freezes the UI.
  // Use await to wait asynchronously for the method to return
  await ProcessDocumentRangeAsync(_startId, _dest);

  // TODO::Do something with the populated list
}

public async Task ProcessDocumentRangeAsync(int startID, string root)
{
  // Execute loop on a background thread
  await Task.Run(() =>
  {
    int year = 28;
    for (int i = startID; i < _endId; i++)
    {
      _instID = year.ToString() + i.ToString().PadLeft(5, '0');
      BuildDocumentList(year, i, _instID, root);
    }
  });
}

private BuildDocumentList(int year, int image, string instID, string root)
{
  string documentIndexFile = root + year.ToString() + "\\" + instID + ".txt";
  if (File.Exists(documentIndexFile))
  {
    return;
  }

  // Because Document extends MainWindow, 
  // we must create the instance on the original Dispatcher thread (UI thread).
  // This is required because MainWindow inherits the DispatcherObject from Window.  
  // DispatcherObject instances have thread affinity (Dispatcher affinity).
  // Creating them on or accessing them from a thread that is not the associated Dispatcher thread
  // will throw a cross-thread exception (InvalidOperationException).
  // To solve this, we must execute the instantiation on the Dispatcher thread using the Dispatcher.
  // We use DispatcherPriority.Background to prevent the loop from exhausting the UI thread.
  // We could improve the performance if we could get rid of the Dispatcher.Invoke,
  // for example by avoiding making Document a Window. 
  // Instead it should be a simple data model (and not a control).
  this.Dispatcher.InvokeAsync(
    () => CreateDocument(year, image), 
    DispatcherPriority.Background);
}

private void CreateDocument(int year, int image)
{
  var newDocument = new Document { Year = year, Image = image };
  _queuedDocuments.Add(newDocument);
}

如果在列表填充后没有任何事情要做，则不必使用await。只需创建后台线程并返回到UI线程并继续：

private async void _btnStartClick(object sender, EventArgs e)
{
  // Call to Method which no longer freezes the UI.
  // Because of we don't await anything here 
  // the method is a pure concurrent method and we can 
  // immediately continue to do work on the current thread.
  ProcessDocumentRange(_startId, _dest);

  // TODO::The list is not populated at this point. 
  // The operation is still executing in parallel-
}

public void ProcessDocumentRange(int startID, string root)
{
  // Execute loop concurrently on a background thread 
  // and return immediately to the caller (click handler) 
  Task.Run(() =>
  {
    int year = 28;
    for (int i = startID; i < _endId; i++)
    {
      _instID = year.ToString() + i.ToString().PadLeft(5, '0');
      BuildDocumentList(year, i, _instID, root);
    }
  });
}

private BuildDocumentList(int year, int image, string instID, string root)
{
  string documentIndexFile = root + year.ToString() + "\\" + instID + ".txt";
  if (File.Exists(documentIndexFile))
  {
    return;
  }

  // Because Document extends MainWindow, 
  // we must create the instance on the original Dispatcher thread (UI thread).
  // This is required because MainWindow inherits the DispatcherObject from Window.  
  // DispatcherObject instances have thread affinity (Dispatcher affinity).
  // Creating them on or accessing them from a thread that is not the associated Dispatcher thread
  // will throw a cross-thread exception (InvalidOperationException).
  // To solve this, we must execute the instantiation on the Dispatcher thread using the Dispatcher.
  // We use DispatcherPriority.Background to prevent the loop from exhausting the UI thread.
  // We could improve the performance if we could get rid of the Dispatcher.Invoke,
  // for example by avoiding making Document a Window. 
  // Instead it should be a simple data model (and not a control).
  this.Dispatcher.InvokeAsync(
    () => CreateDocument(year, image), 
    DispatcherPriority.Background);
}

private void CreateDocument(int year, int image)
{
  var newDocument = new Document { Year = year, Image = image };
  _queuedDocuments.Add(newDocument);
}