I just did a test in a brand new project with this code on the first frame and a symbol in the library with text field.
import flash.display.MovieClip;
import hiResStats.net.hires.debug.Stats;
import flash.display.BitmapData;
import flash.display.Bitmap;
import com.greensock.*;
import flash.events.*;
var stats:Stats = new Stats();
var holder:MovieClip = new MovieClip();
addChild(holder);
for (var i=0; i<1000; i++) {
var newitem:item = new item();
newitem.label.text=String(Math.random());
var bmp:BitmapData=new BitmapData(320,60);
bmp.draw(newitem);
var bitmap:Bitmap=new Bitmap(bmp);
bitmap.y=i*60;
holder.addChild(bitmap);
}
var blitMask:BlitMask=new BlitMask(holder,0,0,320,480,true,true);
addChild(stats);
addEventListener(Event.ENTER_FRAME,go);
function go(e:Event) {
holder.y-=10
}
iPhone 4s ,GPU ,full screen , 60fps
with blitMask on it runs at 55fps(quite noticeable) and sometimes crashes at start up
without blitMask it runs flawlessly at 60fps .