Miren
Thanks for your code, which helps a lot.
In your code, we found two problems,
1. Map unit is not set properly.
Map1.MapUnit = GeographyUnit.Meter
2. Too many markers are displayed in the initial extent which makes IE jammed up (as following)

Here are 2 suggestions for this case:
• If you don’t need popups or context menus for the marker, you can render them using ShapeFileFeatureLayer, instead of drawing every marker on client side, drawing all the points on one image on server and pass it back, that will be much faster and cheaper
• If you want to use markers, try to set their zoomlevels to reduce the count of markers in a view extent. For your case, the map will show better if the marker overlays’ initial zoomlevel is set to 17.
markers.ZoomLevelSet.ZoomLevel17.DefaultMarkerStyle.WebImage = New WebImage("marker_blue.gif")
markers.ZoomLevelSet.ZoomLevel17.ApplyUntilZoomLevel = ApplyUntilZoomLevel.Level20
Ben.