Pre-buffer Bitmapdata

Hey
Before I go any further, I just want to clarify that this is Not about loading.

So with that said. What I have is a .png sequence that will play forwards and backwards on mouse-over/out. I achieve this by having an EnterFrame event hide (visible) the previous image and displaying the next depending on the direction the sequence is playing.

Now to the actual problem. The first time I go through the sequence the framerate drops significantly, at least making it noticeable. Note that I’m simply setting the visibility property and the bitmaps are already preloaded and added to the stage before any playing/reversing is added. Once I have played through the whole sequence, any following playing/reversing is super-smooth.

So what would this be all about?
And are there any tricks that might resolve this?

Edit: Changed to Blitting instead of visibility, question above still stands