I've seen that if you have a manually awarded badge on a course with lots of users (think really really big), it takes a VERY long time to run the cron.

Looking into it:

review_all_criteria() gets all enrolled users, and a list of all users who have already earned the badge.
For all those who haven't earned it, it calls $crit->review(), where $crit is an instance of each of the types of criteria.
In the case of award_criteria_manual (and possibly others), review() runs a get_record().

So for a course with 1,000,000 users, that means we perform 1,000,000 get_records. This is obviously slow.

Even for a small course, this will hammer the DB and kill performance.

We should be able to pass a cached recordset into criteria.