In general IE is slower than Firefox when it comes to rendering speed. Since RadComboBox control is far more complex than the standard dropdown it is expected to be slower as it uses much more built-in JS and skin resources to provide its advanced behavior. However, there are tips and tricks to make the combobox as fast as possible. Such a tip is using the Load On Demand feature that let you load an empty combobox when the page is first loaded. Then, the items can be added as the user types in the input field. This will definitely save loading time and will improve the combobox speed.
In addition, you can use RadGrid with paging. For 1000 records, the RadGrid will take approximately 10 seconds to load, no change in single item select/deselect. We highly recommend using paging when binding to a large amount of data, as this feature significantly increases RadGrid performance.
For more suggestion on optimizing RadGrid performance, please refer to this
Also, I suggest that you refer to the following post on optimizing tips for RadControls:
the Telerik team