Slowdown when importing MovieClips after preloader

As most of us realize, you have to uncheck “import in first frame” to make sure your preloader loads up before your entire flash does… but then you’d have to place your to-be-imported MovieClips somewhere after your preloader. The MovieClips I used were very large in size (dimension-wise and graphically; lots of bitmaps) and for some reason this causes the entire game to slow down A LOT. From a solid 30 FPS to a 17 FPS. For some reason, this slowdown does NOT happen if the MovieClips were “imported in first frame” and without the MovieClips placed in the timeline after the preloader.

The frames holding these MovieClips were never shown. They were skipped using actionscript to go to the next scene. But oddly, these MovieClips are eating up CPU processing power… Perhaps this is supposed to happen, but I’d like to at least know why…

Any insight would be appreciated. Thanks.

(This is not a programming question, but seems to be relevant mostly to only games.)