Jquery element+class selector performance
===

I was hoping `$('#childDiv2 .txtClass')` or `$('#childDiv2 input.txtClass')` perform better when selecting `<input type="text" id="txtID" class="txtClass"/>` element. But according to this http://jsperf.com/selectors-perf/3 performance analysis `$('.txtClass');` is the best selector among this. I'm using JQuery 1.7.2
Does anybody have explanation for this?

*HTML*

    <div class="childDiv2">
        <input type="text" id="txtID" class="txtClass"/>
        <p class="child">Blah Blah Blah</p>
    </div>​

*JS*

    $('.txtClass');
    $('#childDiv2 .txtClass')
    $('#childDiv2 > .txtClass')
    $('input.txtClass')
    $('#childDiv2 input.txtClass')