There is a section in OpenAM documentation - Preparing an External Configuration Data Store, which is useful . Read here.
The most important item is to set indexes on OpenDJ during configuration. See below:
In a recent deployment, I have forgotten to set the above indexes and the machine where OpenDJ servers were running on always hit 90-100% CPU. Very top-ish through the day.
After the indexes were set, the OpenDJ servers went back to normal. Proper indexing does help.