Even if you run runJobs.php non stop, there is a delay.
Users got used to action=purge, but that doesn't help either here.
And there are more problems: https://www.mediawiki.org/w/index.php?title=Topic:Tb8v3cjly5vffl05
To sum it up: After adding a page to a category I click on the category link and the page is not there.
This is inacceptable.
With onPageContentSaveComplete you can grab the categories that changed by an edit. But if you delete related category memcaches, you end up with wrong caches when the user visits a category with freshly flushed cache before runJobs completed categorization.
T125366 would fix it.
What is the performance win anyway? Categorization is done after page saving. No performance win for the user.