It is now working fine for me. I suggest to add onLoad JavaScript handler which will finish checkbox initialization after page load. Example untested code:

<script type="text/javascript">//<![CDATA[
function init() {
  twofastest = document.getElementById('twofastest');
  if ( twofastest ) {
    if ( twofastest.checked ) {
      twofastestClick();
    } else {
      checkBoxClick();
    }
  }
}
window.onload = init;
//]]></script>
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