You should take a look at the UIMovieClip code. In it you’ll see there’s an enterFrameHandler. Basically in the Flex world, we have this nice contract of invalidation/validation and notification of when my size changes (you call invalidateSize()). However, in the Flash world, this doesn’t exist, so every frame UIMovieClip is asking: “hey, have you changed size?”.

I’ll look at how this process can be improved in the future to better optimize these scenarios where you want lots of these flash components. Part of it is that the Flex Component Kit for Flash just wasn’t made for having so many of them at the same time. For now, I’d say you should monkeypatch UIMovieClip.as. Then, modify the file to either set trackSizeChanges to false or to remove the enterFrameHandler all-together.

Hope that helps,
Ryan