Plugins loading is one of the performance big factors,
some installations has lots of plugins.
all active plugins are loaded for every page request!
some of these plugins needed only in admin area, some needed only for specific page/s only. but all of them loads for every request, some with overhead in loading, some even add unwanted content to pages that not related to the plugin. some of these plugins are HUGE.
I've put a hack to selective load of plugins depend on the page request:
1. by default all plugins loaded
2. array of selective plugins with include/exclude sub arrays, each contains a list of reg exp of the URI request.
3. in wp-settings.php in the plugin loading loop just added a one condition test for each plugin calling a validation function if to load it.
I suggest to add to the plugins main page to each plugin an include/exclude input boxes to put comma separated, uri (or page numbers...) regular expression.
My code is working like charm on my site. Its very small, very efficient and the performance improvement is great (depends on how many extra plugins and their size loaded to unwanted pages).
And in the future plugin development codex should include directions and logic/option for each plugin to "tell" where should it be loaded (admin only plugin, page plugin...) as default values for the new added fields. to avoid the need of the users to find out the proper settings.