To reproduce set he following properties:
MouseSelectionHandler.DoubleClickTime = 1000;
MouseSelectionHandler.MouseDragThreshold = 2;
MouseSelectionHandler.MouseDoubleClickThreshold = 60;
Profiling results show bottleneck in the line-breaking logic within Paragraph MeasureOverride.
The image adorner in is not shown in NetCore/6/7/8 when ApplicationTheme is not set (XAML only).
Workaround:
Explicitly set the theme:
StyleManager.ApplicationTheme = new Office2016Theme();