Pretty much any environment where you care about performance of your code, actually. A lot of the simple-looking jquery stuff is implemented with tons of indirection, branching, etc.

Just compare the work a browser has to do for something like:

  document.getElementById("foo").setAttribute("bar", "baz");

to the work needed for the jquery equivalent...

Now the good thing is that most consumers of jquery don't care about performance of their code (in that they're doing small enough amounts of stuff it doesn't matter much).