I am currently experimenting with SEDE. Seems like median time to close answered questions is around 7h, only 4% are below 20min. I'd like to narrow this down to questions with "missing context" close reason, but I cannot find the joins associating a post with its close reason.

I do not know much about SEDE, but if I look at the database scheme, it seems that there is only on close reason for off-topic - if you look at CloseReasonId . Database schema documentation for the public data dump and SEDE