You can maybe go tilebased with the background and draw the tiles into 1 bitmapdata buffer, which you render to the screen.
You can use the same buffer again and draw all sprites onto it, remember you are still just using one screensized bitmapdata to display everything... you have to make sure objects get destroyed when not on screen or not used anymore. You can check if the algorythms and calculations you do are optimized enough, meaning don't do somevar = (2+2+3)/2; when you can go somevar = 3, ect.
But basically if you draw all your stuff into one bitmap its easiest for the renderer and you can draw up to may be 10000 simple objects at the same time and somewhat okay fps?
Are you using a vcam? Don^t ever to that for games if so... ect. could write a book about this but basically it all depends on how you wrote your code and all. Short repetition: Don't use vectors, use bitmaps = faster rendering. Don't use many displayobjects at once -> best to use just 1. :)