Recently migrated forum to a new server. Everything has been working fine as far as everything is known, except for creating a new forum. Trying to do so takes a while, then results in a 504 timeout from the server (On all pages) for a few moments, and the forum isn't created when it recovers.
Well the 504 timeout is coming directly from the server. The rebuild process will be time/CPU (and possibly memory) intensive, but it shouldn't wholly block other connections. If it is, it sounds like only one PHP process is being processed at a time (no task switching), which is likely to give bad performance in general. I'm guessing you're using a FastCGI server so you may want to look at that configuration (the number of processes/children available).
Otherwise, 1.2 would help. It doesn't inherently change the amount of work that needs to be done -- there is a lot of recalculation to do -- but it's designed to allow that work to be split into multiple steps.