if you had over 9000 topics unread in CS, you'll have over 9000 times the average number of posts per topic unread posts in Discourse
Since I didn't have an account in CS, I guess all topics technically are unread for me (in reallity they aren't, since I've been lurking and read quite a lot of it). I hope you will make all imported posts marked as read for users not having an account on CS, or else me and the rest of us Discourse but not CS users will have a lot of clicking and scrolling to do to get reasonable "new" and "unread" counters here...