Time to first response is calculated by subtracting the value of the topic’s created_at field from the value of the created_at field of the topic’s first reply. If you move a post to an existing topic that does not yet have a reply, that post’s created_at field will be used to calculate the response time for the topic. Only ‘regular’ posts are counted as responses - ‘small action’ posts that are added by Discourse are not counted as replies.
For details about the query that is used, have a look at this topic: Time to response.