The easiest way is to do $elt->in( $parent). If you only have the ID of the parent, as it happens, those are stored and you can use $twig->elt_id( $parent_id) to get the element itself.

elt_id is fast, it is just a hash look-up, but in has to walk the ancestors of the element. I would be surprised if this was a real bottleneck in an application though.Does it really cause you performance problems? In that case it would help if you could show us a bit of code (and the associated data).