In the case of querySelectorAll, it does[1] (after performing some checks for easy wins) and if this test[2] is anything to go by, it can actually perform better in some cases.

[1] https://github.com/jquery/sizzle/blob/master/sizzle.js#L237

[2] http://jsperf.com/yui3-vs-jquery-selector-test-3/54