Reported by Sam in Moodle HQ chat:
with moodle 1.9.4 we see a critical performance problem on the root /index.php (or anywhere that draws the site admin block). We've tracked this down to a usage in courses.php of admin_setting_users_with_capability which ends up doing a pretty horrible get_users_with_capability that returns all 500,000 users. This line of code is still there in current MOODLE_19, but I wonder if it's been fixed in any way - does anyone know?
(unfortunately our moodle 1.9 test installs don't have large user numbers)
(ie basically, is this our system being slow, or is there something wrong with our settings for the capability in question, moodle/site:approvecourse)