Yes, we are aware that the unread topics page is super slow, and no, of course that's not what we had in mind. The forum software is not self-built but that doesn't change the fact it's not meeting expectations, and we can blame ourselves for not identifying this at an earlier stage. With our limited resources we have to make fairly big trade-offs on what we spend our time on, and unfortunately sometimes that doesn't work out as well as we had hoped.
I'm confident that everything will work out in time. As you wrote, it's 2017 and there is no reason that the unread topics page should be so slow, so we should be able to fix that. However, as has been mentioned before, it may take a bit longer than we all would want, for the simple reason that NSMB sometimes has to take a backseat when work/life are demanding most of our available time. This is the reality, can't make it any nicer.
In the meantime, keep giving us feedback about what works and what doesn't, it helps a lot.
Thanks for your patience!
BTW If you haven't done so, marking all forums as read helps a little bit (after the migration, all 2.5M posts were unread for all users).