Lots of masks vs lots of images-speed

i am building a sidescrolling advergame with a lot of images in the background that need to disappear to reveal other images. i am realizing that i am not performing at my max framerate. would my game go faster if i:

have one large underlying image and about 30 small images (each in its own movie clip) on top of it that dissapear to reveal the bottom image.

or

have two large images, and use multiple mask movie clips that hide the top image and gradually reveal the bottom image.

i am opperating at low quality
remember, this advergame scrolls and has a ton of extra stuff going on at the same time.

heres more detail if you want to know more about it:
in the game a player collects dance icons and when the player presses a key the character does a dance and images of a boring or drab background are converted into an exciting dance club background. the total background is about 2400 pixels wide, but things are set up so that only the stuff appearing in the background that is supposed to be onscreen is actually there.

thanks
jason oda
starvingeyes.com