I'm not an expert on flash display performance, but I have done some tests and extensive reading on the subject. Generally I've found that using copyPixels() is just about the fastest way to do graphics in Flash. I did a test that showed blitting with bitmaps in Flash was quite a bit faster than creating display objects and moving them around. If you use cacheBitmap it's a little better with display objects, but that only counts if they're not rotating. I suspect the blit method would be faster with rotation, but I haven't done any tests yet. I am working on a simple display engine for Flash, but it is delayed slightly because I'm moving from APE to Box2d. I am hoping to allow optional caching of bitmap rotations for objects that are used a lot.
Never use Movieclips for a display object unless you specifically need the features of a movieclip. This is because there is a lot of overhead involved and you should use Sprites instead for display objects if you need a display object.
Other general performance tips include never using uints, always use int or Number instead because apparently even Numbers are faster than uints contrary to what you would think from previous programming experience. And always type your variables and eat your vegetables.
One final tip before I give you a bunch of useful links: I don't know if this will improve performance to the degree that it did with APE (~30%), but if you use a square root approximation instead of the native sqrt function in Flash it should make a very significant difference in performance with Box2d as well. I will be testing this soon, but I'm still very new to Box2d and just trying to get the hang of things. Here is the link where I found out about the sqrt approximation: http://osflash.org/as3_speed_optimizations
(toward the bottom)
They don't seem to believe their own results, but I tested it with APE and I swear I saw first hand that it makes a huge difference. I even played with the threshold value to make it a rougher approximation than they used and with no visible loss in the accuracy of the simulation. I think I used .002 or something instead of .00001.
And here are all the links I've found useful about optimizing Flash (display or in general) so you can blame the people who originally wrote about them if they're wrong and not me for passing on Flash voodoo magic that turns your teeth green:http://www.danielhai.com/blog/?p=38http://www.danielhai.com/blog/?p=55http://rozengain.com/?postid=35http://www.boostworthy.com/blog/?p=82http://www.bytearray.org/?p=67http://osflash.org/as3_speed_optimizations
(same as from above text, this one's really good)
BTW big thanks to Erin and skatehead for making Box2d and Box2d in Flash possible!