Come on. A "big-board" has 1000-10,000 users online at a time, 10+ million posts. Anything below can be scaled rather effectively just by throwing simple hardware at the problem. At least in my microcosm.
There is a difference between a big board, (frequently classified as having more than 500.000 posts) and a high traffic board with 1.000+ concurrent users online.
In any case, I am happy to read that XenForo has kept big boards and high traffic boards into account. I would be interested to learn about Xenforo performance & scalability.