Hi Pik Lee,
We are currently addressing some performance issues and we will try to do our best to address as many of them as possible for the 2011 Q1 release. We haven't been able to identify any significant changes in the performance of Sitefinity with more users or roles, therefore if you experience something we will be very glad to get them resolved asap.
Thank you for contacting us and please let me know if you have any other questions.
the Telerik team