A very useful feature for large forums is X-Accel-Redirect. Which allows you to solve the problem of the forum, which puts all the work on php, but thanks to this function, you can transfer it to the nginx web server.
Hands down one of the best and most essential XenForo addons if you're planning on making use out of the attachment system. Xon is a very patient developer who made this addon possible, and we're using it on our attachment-centric forum (over 100,000+ files) without any issues whatsoever. If you want your users to be able to stream video files, this is the addon that you want and need!