You can cancel the animation of RadMenu popup by setting the AnimationSelector
Additionally, there is a slight fade animation built-in in the Popup class used by RadMenu. I am afraid this behavior is not customizable as it is built-in.
However, with the upcoming 2012 Q1 release, RadMenu will be updated with a new popup class that does not contain this functionality. You will be able to use the control stripped of any default animations.
I hope this will be helpful.
the Telerik team
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