On Caching As Bitmap

Basically, use cacheAsBitmap (both set with AS2 or set in the Flash Environment Setting) when you want to animate a complex vector inside a MovieClip, but only on the X and Y axis. Scale and rotate is a no-no - it will undo the caching.
More so, shapes that use bitmap fills, and text are both considered complex vectors, so use cacheAsBitmap there as well.
Bitmaps created via AS do not take advantage of being cached as a bitmap.

From Kirupa's forum.