Our client use JOOMLA extensively and hosted on cloud VPS, and there are times where the sites received up to 5000 users at once due to special event like local football match.
So we need to tune the server to accomodate the high concurrent users. We use Varnish-Cache to cache the dynamic page and relocate static contents like CSS,IMAGE,JS to another server. A clever use of RSYNC and .htaccess made this possible. So the load for static contents will be thrown to our NGINX server.
For database, we use MySQL and to optimize the performance, we use MySQL-CACHE. Monitored using MYTOP during the event, MYSQL-CACHE HITS did save us.
And for JOOMLA itself, we only change the SESSION HANDLER from DATABASE to NONE ( file based ).
With more budget, we would relocate everything to AWS and design a more reliable and resilience architecture over there like AWS LoadBalancer, RDS,Route53 and lots more.