The thing that stood out the most to me in Addy Osmani’s slideshow was their performance analysis of Wikipedia’s visual editor. It has some pretty serious performance issues, and it was cool seeing how they were tracked down via Chrome’s dev tools.
And a lot of it was jQuery.
Plain JS is 21x faster.
$elem.style.display = "block"
Plain JS is 51x faster.
You can find lots of examples like this.
The point isn’t that jQuery is bad. Not at all. jQuery has, over and over again, prevented my life from turning into a furious ball of nothing. jQuery is awesome. On many projects the performance difference isn’t going to matter.