I still do some old school JQuery manipulations, and a lot of what kills you on the performance front is also the repeated manipulations of the same set of elements in the same frame. Often, you go in and change an element. Only to have another piece of javascript change it again. Each of those modifications then require a complete relayout of the page, that's when it gets expensive.

You could say that we should just optimize our JQuery, and you'd be right. We just don't have structured way of figuring out everything that touches a "component". (What is a component anyway, when it's all adhoc).