As this is a forum post, I am not sure which version of the Telerik.Web.UI you are using. Over time we have done a number of speed improvements as well as use techniques to make the [client-side] decoration process used by the RadFormDecorator smoother.
In fact, in the coming Q2 2009 we have implemented a new rendering, which is significantly faster than the pre-Q2. The following blog post contains more information about it:
RadFormDecorator Q2 2009 - more decoration options, faster rendering!
On a side note, one general suggestion that should speed up rendering is to set your Web.config's <compilation debug="false">
My suggestion is to test the Q2 once that it is out and let us know if you have problems with performance. Perhaps in this case the best way to continue would be to send us a simple web project which contains your problematic page for us to examine.
the Telerik team