try setting your publish setting to GPU acceleration
I forgot to mention that I am using GPU publish setting and cacheAsBitmap.
Here is an example of what I am talking about:
example .fla file - http://www.mediafire.com/?vvnn6btrcl6y8lp
example .ipa file - http://www.mediafire.com/?lwgput504ejw48g
As soon as the circle is on or around the red box image it will slow the animation tremendiously. This happens with any type of animation over a imported image or drawn image.
enable cacheAsBitmap for your background.