Creating resource dictionary through code behind lead to performance delay

**Problem description:**

My application contains more than 50+ MS and 150 + custom control, each control style have been defined in separate resource dictionary file and merged MS control resource dictionary in custom control resource dictionary file. And when I have merge control resource dictionary into application resources property through code behind. I am facing serve performance delay in creating resource dictionary at code behind.

```
private void Button_Click(object sender, RoutedEventArgs e)
{
stopwatch.Start();
for (int i=0; i <= 100; i++)
{
Application.Current.Resources.MergedDictionaries.Add(new ResourceDictionary() { Source = new Uri("/ResourceDictionary-Performance-issue;component/MSControl/CustomControl.xaml", UriKind.RelativeOrAbsolute) });
}

stopwatch.Stop();

MessageBox.Show("Time delay in adding resource:" + stopwatch.Elapsed.TotalSeconds);
}
```
Also, I have tired to reproduce in simple application. Please find the demo application attached below:

**Sample:**
[ResourceDictionary-Performance-issue.zip](https://github.com/dotnet/wpf/files/8844952/ResourceDictionary-Performance-issue.zip)

Replication procedure:
1. Run the demo.
2. Click button to merge 100 resource dictionaries to current application.
3. It took more than 4.5 sec.

**Actual behavior:**
Performance delay occurs when creating resource dictionary.

**Expected behavior:**
Performance delay should not occurs when creating resource dictionary.

Can anyone suggest a solution regarding this how to avoid performance delay when creating resource dictionary at code behind.