There are a bunch of things to optimize your server/forum:
- Are you using memcached/opcache/xcache? vBulletin 5 should support those modules.
- Are you using Apache as software to run your webserver? nginx is much better at handling lots of connections.
- Are you using MySQL? MariaDB is based on MySQL, a bunch of developers who worked on MySQL now work on MariaDB. I noticed better response times with MariaDB instead of MySQL.
- Did you optimize all of the images of your forum style? You can use
this tool to optimize a bunch of images at the same time.
- Are you on shared web-hosting? Use a VPS/dedicated server and you will definitely notice a difference. Also to do most of the above suggestions you will need a VPS/dedicated server.