For my forums 0 slow queries - usual threshold is 1 to 2 seconds. Enable XF debug mode and see how big a portion of page load time is mysql related and see and profile the slow queries i.e. EXPLAIN to see if it's something that can be improved.
Forums https://community.centminmod.com/ running my own Centmin Mod Nginx LEMP stack with Nginx 1.17.2, PHP 7.2.21, MariaDB on 8GB 4CPU Linode KVM VPS