As you are probably aware, our controls offer rich client-side API and built-in functionalities that you do not get by default with standard asp or html controls. Each of these features makes the control markup heavier and if client-side related - requires extra time to initialize on the client (browser).
This would be mainly observed in IE 6 or Safari, which have various flaws in their box model and problems with handling too much table data at once (especially nested table data). You can go through the "Performance tips and tricks" chapters of each of the control's documentation and use the suggestions there.
If you make use of any ajax framework, you might want to apply a loading panel showing that there is some processing in the background.
the Telerik team