jQuery is so easy to use that sometimes we just forget that it’s not CSS. While using CSS, we don’t have to give much thought to performance, because it’s so fast that it’s not worth the effort to optimize it. But when it comes to the real world, jQuery can drive developers crazy with performance issues. Sometimes you lose precious milliseconds without even noticing it. Also, it’s so easy to forget about some functions and we keep using the old (and not-so-good) ones.