When a new item is created, we have functionality to instant-index it synchronously, even if with partial information, so it appears quickly in the index. However, this does not happen when the item is converted from redirect to non-redirect (this usually happens when merge is being undone). Since we do not index redirected items, the action on un-redirecting is essentially analogous to creating a new item, so we should provide same instant-indexing in this case.
E.g. for https://www.wikidata.org/wiki/Q17292367 redirect was removed at 19:34, 4 October 2018 but it appeared in the index only 10 minutes later, which makes it hard to work with such item, since the search is very basic need for using item in Wikidata UI.