The Brushes declared as resources in XAML are 'unfrozen' by default. For our application we have used a tool which generates styles for all the common controls. But I found that it is also declaring a lot of Brushes (SolidColor, LinerGradient etc.) in resource dictionary which is used by our application.
Is marking them as 'frozen' more performant?
Yes. See the suggestion from Microsoft in this article
A sample from their analysis:
SolidColorBrush: 212 BytesSolidColorBrush: 972 Bytes