For any dynamic website, if it is not the performance of the network/server, database queries are usually the source of performance issues.
They should be optimized for sure and minimized with the help of caching.
phpFastCache is an open source PHP caching library that comes as a single-file and can be integrated so quickly.
It has support for multiple caching methods (apc, memcache, memcached, wincache, files, pdo and mpdo), the period of caching can be defined and has a very simplified API.
Requirements: PHP 5.1+