I have some custom JS in one of my pages where different parts are highlighted as you hover over so you can see associations. There is some processing behind the scenes, but in IE7 as I hover over I see serious delays (not real-time) and the CPU is 100% on one of the cores. With FF3 it goes about to 50%, while Opera barely touches 15-20%.
The RadComboBox you mentioned is heavily dependent on JS; if you have the option of switching over to the browser's combobox then do so as that will have the maximum performance.
Sorry I can't be of more assistance.