Using a framework or custom-built website offers much more freedom when it comes to optimizing certain processes. In order to offer maximum flexibility, CMS are often much more restrictive and include compulsory resource-sapping features.
This is why the CMS Drupal uses the concept of ‘hooks’ for its module system, which requires the functions within modules to follow a specific naming convention. However, testing to see which functions exist is a resource-consuming process, whereas custom-built websites do not need to ‘search’ for these functions as they are already known.

Caching systems should be entirely in memory (RAM). This removes the need to use up CPU power in order to manage input/output from the hard drive.
The aim is two-fold: quickly serve a client request and reduce the amount of hardware (and software) needed for the server to return the response.
Reading/writing to memory is very quick, meaning the time spent consuming resources is particularly short. Moreover, the practice also increases the life span of the components as there is no mechanical movement, unlike when the hard drive is used.