Improving performance with overlapping MCs?

I'm having some performance issues in certain areas of a flash game Im making.

Here's the situation:

I have big movieclip containing a map that can be scrolled and scaled. There are some pannels with a bunch of MC buttons, controls, etc. on the right side of the stage. These are seperate from the map and stay in the same place.

Everything runs fine untill the large map is  zoomed in and/or scrolled such that the right side buttons and stuff are overlapping them map. Then the framerate jumps down from a nice smooth 30 fps to about 3 fps while the map is moving. There are no issues with the map moving when they don't overlap.

Now the thing is, is that's there's no real interaction between the buttons and the map. No transparency or anything, in fact I'm perfectly happy with the map dissapearing behind a side pannel. There are only about 3 pannels that the screen drawing routine would have to worry about, as all the buttons rest on top of the pannels, so why is my performance taking such a hit?

(Hmmm. Actually I do have a lot of transparency, but each is on top of a solid background, and none interact with the map.)