The privacy provider for core_grade calls the following SQL in \core_grades\privacy\provider::get_contexts_for_userid(). On MySQL at least, this is incredibly inefficient as it causes full table scans on grade_grades and grade_grades_history. (We have seen it take over 24 hours to run, with 800,000+ and 15.5 million records in those tables respectively.)

SELECT DISTINCT ctx.id

 FROM {grade_items} gi

 JOIN {context} ctx

 ON ctx.instanceid = gi.courseid

 AND ctx.contextlevel = :courselevel

 LEFT JOIN {grade_grades} gg

 ON gg.itemid = gi.id

 AND (gg.userid = :userid1 OR gg.usermodified = :userid2)

 LEFT JOIN {grade_grades_history} ggh

 ON ggh.itemid = gi.id

 AND (

 ggh.userid = :userid3

 OR ggh.loggeduser = :userid4

 OR ggh.usermodified = :userid5

 )

 WHERE gg.id IS NOT NULL

 OR ggh.id IS NOT NULL

This seems to be down to MySQL being unable to optimise the query to use indexes for all the joins and the filters.