lines and lots of individual pixels always slow down the
flash player. if this is the case try switching each "level" into
its own swf file. when the user is ready to play level one you
would load the first level swf on _"level1" of the flash player.
when the level is finished being played you can load level 2
in its place on _level1 of the flash player
the best way to do this this is have a function on level0,
your background swf, that is call something like changeLevel.
inside that function you can pass it which level you which to
change to and it will unload "_level1" of the flash player and
reload it with a new swf, secondLevel.swf.
this is how i make all my sites, it keeps the cache of the
browser very low and thus making the cpu strain low.
if you are confused think of it as making layers inside you
main swf/background swf with other swf's on top of it. only your
background swf really just contains code and maybe a header/logo.
if you'd like further assistance feel free to contact me