I've seen jQuery code. It doesn't do anything particularly useful for me.
It solves problems I don't have. I don't need to lookup elements in the dom, because I put them there. I already know where they are.
I don't need a slow .bind() method - I know how to write closures.
Look at one of the top news items - jQuery plugin 'behavior'. It just doesn't make any sense.
Look at some of the source code. For example .addClass(). It's the most general you could imagine - can take a list of classes, or a function. Yet I'm betting most of the time it's being called with a single class to add. There's a lot of wasted checks and effort in there. It's checking the type, it's splitting the string on " ", it's checking to see the class isn't already listed etc etc
Also everything is assuming it's operating on a list of elements - cue wasteful for loops when they're not needed.
So, when you do the seemingly simple
$('#myelement').addClass("foo");
it'll be cycling through the array of elements of length 1, splitting 'foo' on spaces, checking if 'foo' is a function, etc etc etc etc
Those are precious wasted cycles. And for what? so you can write:
$('#myelement').addClass("foo");
instead of:
// I already have a js ref to myElement thanks,
// and know myElement doesn't already have 'foo' in className
myElement.className+=" foo";
