Entering the settings dialog of xfwm4 (Window Management) triggers the retrieval of themes. However, this happens synchronously which leads to blocking of the dialog until Xfwm4 is done traversing all relevant directories and files within them.
One possible solution to this could be showing the dialog immediately, setting the themes list insensitive and filling it asynchronously. Alternatively GtkSpinner could be used to indicate activity first followed by insertion of the actual themes list after the traversal is done. The latter would not require asynchronous traversal.