AS3 What causes lag with side scrollers? - actionscript-3

Question

I've been programming a side scroller based on a tutorial found in a book. My friend did the same, and his is working perfectly.
I've only really changed a few variable names around (I've also done animations differently) but for some reason, when my character is moving, there is a large amount of lag.
HOwever the lag is only present when there are 'walls' on the stage. When I scroll past them, the lag goes away, then returns.
Walls and Floors both use the same code (they are both assigned as 'floorObjects' variables) and use the same collision code, however I cannot figure out why there is lag involved.
From where the character starts (about 60x) if the character goes left, there is a HUGE amount of lag. If I go right, there isn't too much lag, until the screen starts to scroll.
The lag from going left I believe may have something to do with the program being preventing from scrolling off the map etc. But I can't figure out why there is lag when trying to move right.
I've listed the Scroll code, and the main loop, if need be I can upload the collisions code, and any help would be greatly appreciated.
Scroll code;
public function scrollGame()
{
var stagePosition:Number = gameLevel.x + player.mc.x;
var rightEdge:Number = stage.stageWidth - edgeDistance;
var leftEdge:Number = edgeDistance;
//Scroll the GameLevel to the Left if player moves right
if(stagePosition > rightEdge)
{
gameLevel.x -= (stagePosition - rightEdge);
//Prevent the game scrolling off the stage
if (gameLevel.x < -(gameLevel.width-stage.stageWidth))
gameLevel.x = -(gameLevel.width-stage.stageWidth);
}
//Scroll the GameLevel to the right if player moves left
if(stagePosition < leftEdge)
{
gameLevel.x += (leftEdge - stagePosition);
//Prevent the game scrolling off the stage
if(gameLevel.x > 0)
gameLevel.x = 0;
}
}
Main Loop:
public function gameLoop(e:Event)
{
//Get Time Difference
if(lastTime == 0) lastTime = getTimer();
var timeDiff:int = getTimer() - lastTime;
lastTime += timeDiff;
//Game Cycle tasks
//Only perform if in play mode
if(gameMode == "play")
{
moveCharacter(player, timeDiff);
moveEnemies(timeDiff);
checkCollisions();
scrollGame();
}
}
UPDATE:
So I "profiled" it, most of the time is being spent either in the MoveCharacter() function, using the gotoAndStop() command. So I removed that, and it made no difference, still lagging. I then removed the enemies also, still lagging. But turning quality down to low has somehow fixed it (though at a poor quality now ) Any ideas as to what is causing the lag and how to fix it?

Answer 1

this is from the flash university book isn't it?
The code is fine.
I know what will lag your flash game.
This is a guess though, and I do get this error some times.
Make sure your images are optimized!
If they're imported from photo shop or illustratro then flash will have to deal with those complicate vector points.
Use .png for transparent images, bitmaps don't hurt either.