I've tried to switch every control as soon as the Prometheus version becomes available. It's been my experience that the Prometheus controls never work any slower, but the speed increases vary by control. The editor has by far the biggest improvement in speed, and so far I've noticed a slight performance improvement on the client with the calendar.
My guess would be that it would be difficult for the telerik team to make huge improvements with the performance of the Prometheus controls simply because the ASP.NET ones already work so well. Hope that helps,