Indeed, there is a bug with the initial animation and it always plays for 900ms. We have resolved the problem, and the fix will be available in the next internal build (and the official service pack).
A work-around is to use the animation.duration configuration option, like this:
duration: 10 // milliseconds that the animation will take to play
The (undocumented) animation.duration is about to be replaced by animation.speed -- this is why it wasn't documented in the first place and is unsupported.
Your Telerik points have been updated for the bug report.