In WPF if you choose simple marker overlay, each marker should be one control, so it will be so slow if the number is big.
Please try the InMemoryMarkerOverlay or if you just need shows the features like a marker, InmemoryFeatureLayer is enough. You just need to set the style like a marker:
inMemoryFeatureLayer.ZoomLevelSet.ZoomLevel01.DefaultPointStyle = new PointStyle(“Your Marker Image”);
And here is some sample code to edit a shape file:
ShapeFileFeatureLayer layer = new ShapeFileFeatureLayer("");
Wish it’s helpful.