There are multiple ways to optimize web page, loading speed. If an application depends on MySQL, one of the ways is to configure and use MySQL built-in query caching mechanism. It's a memory based(RAM) caching system.
What is a Symfony flex ? From the official docs you can find that it is a tool used in newer versions of Symfony (default from Symfony 4, but also available from Symfony 3.3) to manage and install Symfony apps and it's bundles....