On Moodles that have a lot of course categories, and a filter enabled for content and headings (e.g. multilang), there are currently excessively many database queries being made (e.g. 1 per course category). For example, if you have 500 course categories, which is often the case for large institutions, you will have > 500 queries on course/index.php or course/category.php.

The database queries are caused by a call to get the context instance for each course category.

I've coded something in the branch https://github.com/brki/moodle/tree/mdl23-preload-filters-for-course-categories . The commit https://github.com/brki/moodle/commit/da23cce06d7edbd2cad43ba9c25396dcf5bfac35 preloads all the course category context instances for course/index.php and course/category.php. This reduces the database queries significantly (it makes two database queries to get all of the course category contexts instead of one for every course category).