setInterval在ie中执行特别慢 是什么问题(尤其ie6下)

Question

function reduseDialog(){
  var that=$('.uploadlayer');
  opac=1;
  filepos=$('.login_info').position();
  tol=Math.floor(filepos.left);tot=Math.floor(filepos.top)+($('.login_info').get(0).offsetHeight);
  dispos=that.position();
  froml=Math.floor(dispos.left);fromt=Math.floor(dispos.top);
  differencel=froml;
  differencet=fromt;
  $('.blackout_box').hide();
  reduceauto=setInterval(reducepos,10);
}
function reducepos(){
  differencel=differencel+5;
  differencet=differencet-3;
  opac=opac-0.01;
  if(differencet<=tot || opac==0){
  $(".uploadlayer").hide();
  //state 0 不显示  1 正在上传  2上传完成
  $('#upfile').addClass('uing').attr('state',1).html('正在上传');
  clearInterval(reduceauto);
  return false;
  }
  $(".uploadlayer").css({'left':differencel+'px','top':differencet+'px','opacity':opac});
  //if(increaseauto){clearTimeout(increaseauto); increaseauto=setInterval(increasepos,10);}
}