In answer to the question in the title, yes the update discourse topic feature is (effectively) immediate. It doesn’t have a queue.
Did it update instantly as part of the second test, but not as part of the first?
It sounds like you’re having some other issues, either in Wordpress or your various caching systems. Is there a particular problem you’re trying to solve here? Is there a scenario in which you need to make multiple rapid updates to the topic?