s2Member has been upgraded and may help with your speed issue.
http://www.primothemes.com/readme/3679/#rm-changelog1.5
(s2Member/s2Member Pro). Speed Optimizations. s2Member's entire codebase has been re-organized into PHP classes containing s2Member's static functions ( dev note: all of s2Member's Hooks/Filters remain as they were ). This new infrastructure allows s2Member to take full advantage of PHP's built-in SPL Autoload extension. This means s2Member's source code is loaded ( only on-demand ) as function calls are made within core routines. So instead of loading s2Member's entire codebase into WordPress®; only the objects/methods needed during the processing of particular page will be included. Long story short, this release of s2Member is much faster than previous versions. For advanced site owners, this will make it more feasible to run s2Member in concert many other plugins; even on shared hosting.