As described in /lib/gradelib.php on lines 1560 (MOODLE_33_STABLE)

the TODO for "do not run this cleanup every cron invocation" appears to fire gradehistory cleanup tasks on every CRON instance. 

Where cases of sites include large numbers of Grade History records - this will cause Grade History Clean-Up tasks to fire every-time the cron fires at the system-level. This can cause large performance lag in some site.

Either, this should be wrapped with a "Run Task X per day" wrapper or forked out of the cron task - into its own cron task.