I have used the latest Q3 RadCombo with more than 2000 items in my project. The search speed in IE is too slow while comparing 2010 Q3 release. But it doesn't have any issues in Chrome and FF. When will it be fixed?
If this is crucial to your application, I'd suggest that you open a support ticket with Telerik. This will expedite an investigation. Alternatively, you can add your issue/observations to their Public Issue Tracker (PITS) to suggest that they investigate a fix.