Incredible loss of performance when encapsulating a DOM element in jQuery (1.7.2)

I was working on a pretty nice task that involved translating the text in a page in real time. For this I created a one page function that would do magic on elements that were added or changed in the page. On specific pages it moved with abysmal speed and I had no idea why. So I went to profile the thing and I was shocked to see that the problem did not come from my long piece of code, but from a simple encapsulation of an element in a jQuery object. I was using it only to have a nicer interface for getting the name of the element and changing an attribute. Here is the code:

var j=jQuery(elem);
if (j.is('img[alt]')) {
   j.attr('alt',translate(j.attr('alt')));
}

Replaced it with:

if (/^img$/i.test(elem.tagName)) {
  var alt=elem.getAttribute('alt');
  if (alt) {
    elem.setAttribute('alt',translate(alt));
  }
}

And it worked very fast indeed. The element might have been body so maybe the encapsulation tries to also parse the children or something like that or perhaps the problem was fixed with later versions of the library. However, think about how many times we used this kind of code without thinking twice about it. Think twice about it! :)