This reproduces the issue described in this WPF bug report.
See the comments in Styles\Styles.xaml for details about how this project reproduces the conditions that cause this WPF performance bottleneck.
When running the application, the right hand side of the view contains elements that control and diagnose the problem. The left hand side contains the elements that cause deferred resource references to be accumulated.
The Count: field uses reflection to display the total number of elements in the internal
_deferredResourceReferences field of every resource dictionary merged into the application’s resources.
See This MahApps issue for details of how this problem is especially evident when using MahApps. The mahapps branch in this repo contains somewhat simpler code that reproduces the problem. Comment out the references to the MahApps resources in App.xaml to compare performance with the standard WPF/Windows styles.