Also you might want investigate enabling performance statistics in PG itself. This will let you track disk page reads and the like ... what buffer size have you set in PG? 21,000 users is a *lot*, undoubtably our largest PG .LRN installation, maybe our largest .LRN installation altogether so I'm not terribly surprised to learn we may have some more tuning to do.
Do turn on permission caching BTW.
