Frame rate

Frame rate on the iPhone just reared it's ugly head. I wrote a function that would allow the user to change the opacity of a given layer of a TMX tile map, but that requires changing the opacity of each of the tiles (sprites) in the layer.
But once I did that, the frame rate dropped to something like 3 fps. So what I am finding I need to do is just change the tiles on the screen at the moment. That takes just 0.001 seconds, since I am updating 70 tiles instead of 2500. The problem is that now I need to update the opacity of tiles whenever I move new ones onto the screen. It'll be a hassle, but it should be much faster.
Anybody have a better idea?

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