Slow framerate in preview; runs fine in browser or through preloader -?

Here’s a problem that I’m stuck on, especially since it defies logic: When I preview my SWF from within Flash or run the exported swf outside of Flash, it has suddenly become extremely slow, maybe 10 FPS or something.

But then it runs smoothly if I

  • run it in a browser embedded in a HTML page
  • run a preloader locally which simply loads the problematic SWF file; once the problematic SWF file shows up, it runs perfectly.

This doesn’t make sense to me, and it’s really annoying to be unable to just Shift+Enter to preview. Anyone tried anything like this before?