[devote]

Я что прочитал...

Ты не ту доку читаешь =)))

вот вырезка с офицального сайта: http://api.jquery.com/first-selector/

Because :first is a jQuery extension and not part of the CSS specification, queries using :first cannot take advantage of the performance boost provided by the native DOM querySelectorAll() method. To achieve the best performance when using :first to select elements, first select the elements using a pure CSS selector, then use .filter(":first").