You can try to build linear gradient brushes between points like the arrows on the screenshot.
I have attached a sample solution that calculates CreateLinearGradientBrush between two locations. The example adds MapLine instance by mouse click event and shows it as a gradient from green to brown.
I hope it helps.
You cannot increase the performance of the RadMap with a lot of objects in the InformationLayer.
I would like to recommend using the Dynamic layer instead of Information layer.
It allows decreasing the quantity of objects on the map at the same time. Also it allows displaying different shapes according to zoom level of map control.
For more information please see the Using dynamic layer documentation topic:
Also there is an Items Virtualization example in our demos:
The example uses WCF service. You can find the sample code of this service using the local path like the following:
C:\Program Files\Telerik\RadControls for Silverlight Q1 2010\Demos\ExamplesWeb\RadMapDataService.svc.cs
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