Why does this script going slower and slower?

programming flash
1

try this and give me response

var max:Number = 50;
var size:Number = Stage.width;
var step:Number = size/max;
var numb:Number = 0;
for (a=1; a<51; a++) {
    this.createEmptyMovieClip(["line"+a], 51-a);
    this["line"+a]._x = 250;
    this["line"+a]._y = 250;
    this["line"+a].lineStyle(1, 0x000000);
    this["line"+a].moveTo(a*step, -(a*step));
    this["line"+a].beginFill(Math.floor(Math.random()*999999 - 100000) - 100000)
    this["line"+a].lineTo(a*step, a*step);
    this["line"+a].lineTo(-(a*step), a*step);
    this["line"+a].lineTo(-(a*step), -(a*step));
    this["line"+a].lineTo(a*step, -(a*step));
    this["line"+a].endFill();
}
onEnterFrame = function () {
    if (numb == 50) {
        numb = 0;
    }
    numb += 1;
    this["line"+numb]._rotation += numb
    
    
};