The downloadable Plugin Open Selected Parts has been updated, and it should now be considerably faster for larger scores.
It should be quite fast if only the full scores is open. If a lot of parts are open it will take more time for the dialog to come up, but it should still be much faster than it was previously.
The order of the parts in the list might be slightly different for parts that contain multiple instruments, but for the the most part the list should be quite similar to the one used by the + button or the Extract Parts list.
Thanks to Philip Rothman who pointed out the speed issue.