Re: [whatwg] Proposal: Adding methods like getElementById and getElementsByTagName to DocumentFragments

On Fri, Oct 18, 2013 at 11:09 AM, James Greene <james.m.greene@gmail.com> wrote:
> I would also love to see `getElementById` added to the HTMLElement/Element
> interface. It would be nice to capitalize on that potential perf boost in
> jQuery as well.

There's no perf boost available for searching by id on an arbitrary
element.  The reason you may get a better perf for the normal
functions is that documents cache a map from id->element on
themselves, so you just have to do a fast hash-lookup.  Arbitrary
elements don't have this map (it would be way too much memory cost),
so it'll fall back to a standard tree search, exactly as a
querySelector would.

~TJ