Silverlight/WPF rendering is slow in general, and when there is a Graph item it is drawn as a Canvas with multiple elements. You can check what XAMl is rendered for a single Graph by running the code from the Exporting Report Programmatically
article (Exporting a report to a multi document format
) for the XAMLInteractive
You can test to use another viewer suitable for web application
as the HTML5 Report Viewer
, where in browser you will get the report rendered in HTML format
(Graphs are rendered as SVG elements or IMG in older browsers).
If you prefer to keep the Silverlight ReportViewer, you can test to reduce the number of rendered data points (data), and extend the timeouts at both client and server - How to: Increase the timeout value of a WCF service
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