Colin Eberhardt kindly had a look at Visiblox Charts and did a little comparison between it and the Silverlight Toolkit Charts. You can read his findings at http://www.scottlogic.co.uk/blog/colin/2010/11/visiblox-charts-vs-silverlight-toolkit-charts-a-test-of-performance-2/.
One of the things Colin says is that according to his measurements Visiblox Charts is 50-100 times faster than Silverlight Toolkit Charts, which sounds about right to us! One thing that Colin doesn't explicitly say, but which is pretty evident from his code snippets is that the API for Visiblox Charts also looks much cleaner. The XAML is about half the length from the Silverlight Toolkit equivalent.
As Microsoft's David Anson points out in his response, the Silverlight Toolkit Charts haven't been optimised for performance, which is a valid point, but that's one of the places where Visiblox Charts excels! He does suggest some alternative approaches and it would be great to see how they stack up.