As of now, if gradehistorylifetime is set, the grade history tables are getting pruned every cron run. This may be a performance issue when huge tables (e,g. 10 or 20 million rows) are getting pruned every 5 minutes. There's a TODO comment in code in this regard, which looks as follows:

//TODO: do not run this cleanup every cron invocation

I suggest that this be re-done as a nightly "cleanup" scheduled task.
Will provide with a code change when completed.