Late to the game, but I just recently installed Flarum, and got it working.
Did a bit of load testing today on a high performance VPS instance running at 3.8ghz CPU's.
Using an NGINX + php-fpm stack.
With only a single php worker, was getting around 32 requests per second on the flarum home page.
With 2 workers... as expected, 64 requests/sec. This is on a 2 CPU box.
This is on a fairly large site with a quarter of a million posts and 9000 user accounts.