OK. I have about 20 looping, animated movieclips inside a big container mc, then on the root I have 4 movieclips and the main code uses about 20 iterations per frame. The background animation makes the game run dreadfully slow (about 10-15fps where 29 without them).

They are just normal drawings using motion tweens to move and loop continuously, could there be a reason for this or is flash that bad at handling this kind of stuff.