Will s2Member slow down my site?
Highly Organized Codebase w/ SPL Class Autoloading
s2Member has quite a large codebase with a lot of powerful features and flexibility. With so much to maintain (and more importantly), with so many files to be loaded into WordPress; it becomes extremely important for the application to be optimized in every way possible.
One of the most important things that the s2Member codebase deals with is OOP design combined with PHP Autoloading. Unlike many other themes/plugins for WordPress, s2Member makes extensive use of SPL Autoloading functionality in recent versions of PHP. In this way, the only parts of s2Member that are loaded at runtime, are those which are required to perform a specific action that is being requested of s2Member.
In short, only a small handful of s2Member's codebase is actually loaded at any given time. This reduces s2Member's footprint down to a bare minimum. Many sites running the s2Member Pro software can do so with just 32M of RAM. That covers WordPress, s2Member, a custom theme, and many other plugins too. Having said that, it is suggested that you dedicate at least 64MB of RAM to your installation of WordPress—just to give it plenty to work with. See this article if you'd like to learn more.