I am using Jquery to make an image scroll across my page horizontally. The only problem is that it uses a serious amount of cpu usage. Up to 100% on a single core laptop in firefox. What could cause this???

Jquery

<script>
    jQuery(document).ready(function() {

    $(".speech").animate({backgroundPosition: "-6000px 0px"}, 400000, null);
    });

    </script>

CSS

.speech {
    /*position:fixed;*/
    top:0;
    left:0px;
    height:400px;
    width:100%;
    z-index:-1;
    background:url(/images/speech.png) -300px -500px repeat-x;
    margin-right: auto;
    margin-left: auto;
    position: fixed;
}

HTML

<div class="speech"></div>