I have experimented with graphs recently. It seems to me that rendering performance is fine. What gets slow for big amount of data is actually building the data structure that support the chart.
Suggestion: Have you tried to preallocate your data points? That helped (not much though) in my test app.
Suggestion for TELERIK devs: Series objects do NOT have a capacity parameter for their constructors. This may be an issue if these collections are based on growable arrays, such as List<>. When the client code adds data points, the corresponding collections will be copied into a new one, the old one thrown away, which takes plenty of time and creates additional memory pressure and needless runs of the GC.