This code makes Flash Player run slowly, why?

[color=black]Hey All,[/color]
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[color=black]I am trying to get my mc to move up and have copies of that same mc follow about every 40 pixels. Basically a scroll with my “8ball” mc. Showing the mc every 30 pixels in the y-direction. [/color]
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[color=black]I am using this code in the mc and I get a message saying that the script is causing Flash Player to run slowly. And that my computer may be unresponsive. Then it asks if I want to abort.[/color]
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[color=black]Any suggestions?[/color]
[color=black]Thanks.[/color]
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[color=blue]onClipEvent(enterFrame) {
speed = -1;
this._y += speed;
}[/color]
[color=blue]onClipEvent (load){
for (i=0;i<10;i++) {
this.duplicateMovieClip(“ball”+i, i);
i++;
} [/color]