Change span text is very slow

Change span text is very slow

This is my code below:
<script type='text/javascript'>//<![CDATA[
$(window).load(function(){
var spans = document.getElementsByTagName("span");
for(var i=0;i<spans.length; i++) {
if(spans[i].innerHTML == "Test 2") {
spans[i].innerHTML = "some example text";
break;
}
}
});//]]>
</script>
<div class="sptests">
<ul>
<li id="sp_test" class=""><span>Test</span></li>
<li id="sp_test2" class=""><span>Test 2</span></li>
<li id="sp_test3" class=""><span>Test 3</span></li>
</ul>
</div>
I want to change span text of li id="sp_test2". I used above javascript,
but it's taking long time to change i.e it is first showing "Test 2" text
then after page loading completes it shows changed text "some example
text".
I want to change that span text with immediate effect on page loading.
Anyone plz helpme...Alternative methods also suggested.