I already answered your support ticket, but I will paste my answer here in case someone else has the same question:
You can completely turn off the animations in RadComboBox (as in most of the Telerik controls) by setting the AnimationManager.IsAnimationEnabled attached property:
<telerik:RadComboBox telerik:AnimationManager.IsAnimationEnabled="false" ... />
There is also a property of AnimationManager that completely disables the animations across the application (should be set somewhere in the code-behind, for example in App.xaml.cs):
In addition, you could customize the animations' spped and easings of the controls that support Telerik AnimationManager by attaching an AnimationSelector. You could see how it's done in our control styles.
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