I've done some superficial speed tests that could be found at http://pastebin.com/f561045be

It turns out, that possibly nextUntil isn't as fast as it could be. See pastebin or attachment for details.

For example the code

var obj = $(this).parent().children();
var start = obj.index(this) + 1;
var end   = obj.index($(this).nextAll('h1:first'));
if ( end == -1 ) {
	slice = obj.slice(start);
} else {
	slice = obj.slice(start,end);
}

is ~3 times faster than

slice = $(this).nextUntil('h1');

I suggest to perform some more tests, especially on large data sets.

I suggest on making some more tests esp. on larger data sets.