Flex mobile - increase list scrolling performance
===

Imagine a tablet app that displays two content areas side by side. They completely fill the display, so are 100% in height and 50% in width.
Lets assume we add a list to one container. Naturally this list will consume half the space of the whole display.

Now to my problem, is it possible that high framerate scrolling is kind of impossible with lists of this size? I've got the most basic AS3 ItemRenderer and still can't get anything higher than 30fps during scrolling. Now the odd part, if I add stuff the other container, lets say another list or other components, the list scrolling performance drops to the low 20s.
So nowhere near the 40+ fps you see Adobe advertising in their MAX shows.

I'm testing on a iPad2 and 3 and even with static values, scrolling isn't really good. Now if I enable streaming values so that the ItemRenderer's `set data` method is called, the framerate drops another 2 to 3 frames.

My (almost) complete renderer looks like this, but even if I strip it to just display a single textfield, disable the stuff going on in the `set data` and also set only the size of the single textfield in the `layoutContents`, performance is as described, about 30 if the list is displayed alone, low 20s if other stuff is displayed as well.

        //FCStyleableTextField is just a StyleableTextField with an additional ID

        private var _textFields:Vector.<FCStyleableTextField>;
		private var _oldValues:Dictionary;
		private var _sym:Symbol;
		
		public function GridRenderer() {
			super();
			_textFields = new Vector.<FCStyleableTextField>();
			_oldValues = new Dictionary();
		}
		
		override protected function createChildren():void {
			
			var _symLabel:FCStyleableTextField = new FCStyleableTextField();
			_symLabel.editable = false;
			_symLabel.selectable = false;
			_symLabel.multiline = false;
			_symLabel.id="sym";
			_symLabel.setStyle("fontSize", fontSize);
			_symLabel.textColor = 0xc0c0c0;
			_textFields.push(_symLabel);
			addChild(_symLabel);
			
			var fidLen:int = fids.length;
			for (var i:int = 0; i<fidLen; i++) {
				var _fid_lbl:FCStyleableTextField = new FCStyleableTextField();
				_fid_lbl.selectable = false;
				_fid_lbl.editable = false;
				_fid_lbl.multiline = false;
				_fid_lbl.id = String(fids[i]);
				_fid_lbl.textColor = 0xc0c0c0;
				_fid_lbl.setStyle("textAlign", "right");
				_fid_lbl.setStyle("fontSize", fontSize);
				_fid_lbl.text = " ";
				_textFields.push(_fid_lbl);
				addChild(_fid_lbl);
				
				if(i>visibleColumns) {
					_fid_lbl.includeInLayout = false;
					_fid_lbl.visible = false;
				}
			}
		}

		
		override public function set data(value:Object):void {
			if(!value) return;
			
			if(data) {
				// check if the value's symbolName is different than the current
				// data's symbolName, if so, the itemRenderer has been 
				// recycled, thus we need to reset the old fid values
				if((value as Symbol).symbolName != (data as Symbol).symbolName)
					_oldValues = new Dictionary();
			}
			
			super.data = value;
			
			_sym = data as Symbol;
			
			try {
				var textLen:int = _textFields.length;
				for (var i:int = 0; i<textLen;i++) {
					var lbl:FCStyleableTextField = _textFields[i];
					if(lbl.id == "sym") {
						lbl.text = _sym.symbolName;
						lbl.truncateToFit();
					} else {
						if(lbl.id == _sym.fidList.fidMap[lbl.id].fidId && lbl.text != _sym.fidList.fidMap[lbl.id].fieldValue) {
							var time:int = new Date().time;
							var timerName:String = _sym.symbolName+","+lbl.id+","+grid;
							globalTimer.addTimer(timerName, time, "reset", lbl, null, null);
							
							var _oldVal:* = _oldValues[lbl.id];
							var _newVal:* = _sym.fidList.fidMap[lbl.id].fieldValue;
							
							// basic color formatting
							if(Number(_newVal) > Number(_oldVal))
								lbl.textColor = 0x40c040;
							else if(Number(_newVal) < Number(_oldVal))
								lbl.textColor = 0xf05050;
							
							// add + to change and changePercent fids if value is positive
							if(lbl.id == "56") {
								if(_newVal >0)
									lbl.text = "+" + _newVal;
								else
									lbl.text = String(_newVal);
							} else if(lbl.id == "11") {
								if(_newVal >0)
									lbl.text = "+" + _newVal;
								else 
									lbl.text = String(_newVal);
							} else 
								lbl.text = String(_newVal);
							
							if(!_sym.fidList.fidMap[lbl.id].fieldValue)
								lbl.text =" ";
							
							_oldValues[lbl.id] = _newVal;
						} 
					}
					
					lbl.truncateToFit();
				}		
			} catch (e:Error) { /* nothing to do here -> try/catch required due to async symbolassembly */ }
		} 
		
		override protected function layoutContents(unscaledWidth:Number, unscaledHeight:Number):void {
			var viewWidth:Number  = unscaledWidth  - paddingLeft - paddingRight;
			var viewHeight:Number = unscaledHeight - paddingTop  - paddingBottom;
			var _previousLabel:FCStyleableTextField;
			var textLen:int = _textFields.length;
			
			for(var i:int =0; i<textLen;i++) {
				var lbl:FCStyleableTextField = _textFields[i];
				graphics.beginFill(0x808080, .3);
				lbl.height = viewHeight;
				lbl.y = paddingTop;
				
				if(lbl.id=="sym") {
					lbl.width = 95;
				} else if (lbl.id == "35000") {
					lbl.width = 24;
				} else { 
					lbl.width = optimalColWidth;
				}
				
				_previousLabel ? lbl.x = (_previousLabel.x + _previousLabel.width): lbl.x = paddingLeft;
				graphics.drawRect(lbl.x+lbl.width, 1, 1, unscaledHeight-1);
				
				lbl.commitStyles();
				_previousLabel = lbl;
				graphics.endFill();
			}
		}


Still, I'm pretty sure that it is not the item renderer that causes the slowdown, cause as I said, it costs 2, maybe 3 frames compared to a renderer that displays just a single textfield.
I rather think that Flex somehow can't handle the amount of vectors being displayed at once, is something like that possible? And is there any way to boost performance?
I already disabled live streaming values as soon as the user scrolls the list, so that flex basically just has to scroll bitmaps (since LabelItemRenderer automatically enables `cacheasbitmap`), but that gained maybe 4 frames. 
What are your guys tricks to make scrolling a little smoother?
