The weekend I had time to do some serialization performance changes. The new beta version that is available here [1] is about three times faster than the current release version [2]. I have done some changes to the IJavaScriptConverter interface (method Serialize) to support the StringBuilder like it was in earlier days with Ajax.NET (not AjaxPro). There are some more changes done to improve the serialization from .NET types to JSON.
If you found an error or bug please report this to the CodePlex issue tracker [3] or discuss this in the AjaxPro Google group [4].