30+ small modules versus 5 large modules versus 1 HUGE module: Which is better? So, we have a large, complex Drupal 6 site, which has lots of secret sauce and magic glue making it work. 

Where we can, we've contributed modules back and made them generic, but we still have 30 or so custom modules to keep it working the way we need it to. Each module (in most cases) has a specific function e.g. our hook alters for the notification system.

We are doing a huge code review at the moment and looking at improvements we can make. One thing suggested is to merge the large number of modules into a few smaller master modules, with lots of include files. We would, perhaps group by general site area e.g. "community", "admin" and "messaging", instead of specific modules per functionality.


Is the effort in doing this worth it? Will it speed up the site?
What module grouping would you recommend?

by site area ("community", "admin", "upload")
by hook ("form", "node", "user", "theme")
everything in one module
something else?

What naming conventions should we use for the include files to keep the codebase sane?
Am i right in assuming the goal should be tiny .module file with includes per hook / menu / area?
It sounds like you don't really know what your bottleneck is. This restructuring will likely have very little impact on performance (unless you happen to find and trim some major bloat in the process), but it may make things easier for you to maintain. If you do decide to restructure I would recommend splitting up along "site features" (blog, image galleries, etc.).

But if performance is your concern you first have to diagnose the problem before you attempt surgery.