[KPaul]

Используй связку onEnterFrame(или setInterval) и for.

this.maxIteration = 100000;
this.curIteration = 5000;
this.count = 0;
this.maxCount = this.maxIteration/this.curIteration;
this.onEnterFrame = function(){
    var p = (100*count)/this.maxCount;
    preload_bar._xscale = p;
    for(var i=0; i<this.curIteration; ++i){
        //...
    }
    this.count++;
    if(this.count == this.maxCount) delete this.onEnterFrame;
}

сорри не тестил...