See more details in the forum discussion http://moodle.org/mod/forum/discuss.php?d=138337
Basically by default, Moodle 1.9.x will set up a cron job that would run this following query every 15 minutes to clean out the "grade history" table. This doesn't cause any site slow or serious bump in CPU but it's still not a efficient query and things could get worse (query > 2 - 3 seconds) when the table gets really big. According to Tim, this query is related to the gradehistorylifetime setting on the cleanup page. I'm filing a bug report here and I should be able to come up with a patch shortly. Stay tuned.