The latest version of the RadControls compiles all the separate controls
into one dll file so that really does help performance as it reduces the
number of calls between dlls. They have also introduced a couple of
wonderful features to help combine scripts used by the various controls on a
page into one file (making them much lighter and far less repetetive) as
well as combining css stylesheets for the various controls used in a page.

A few good performance articles from Todd Anglin, a Telerik employee and
their best evangelist are:

Ajax manager optimization
http://telerikwatch.com/2008/05/optimization-tips-using.html

general manager optimization
http://telerikwatch.com/2008/04/optimization-tipsthe-rad-managers-for.html

Their documentation for the RadScriptManager is at:
http://www.telerik.com/help/radcontrols/prometheus/?RadScriptManager.html
and for the RadStyleSheetManager is at:
http://www.telerik.com/help/radcontrols/prometheus/?RadStyleSheetManager.html

The managers really do help in trimming the page down as well as caching the
data for better performance on subsequent hits.

Hope this helps,
Mark Fitzpatrick
Microsoft MVP - Expression