When a student makes a data request, the grade privacy provider creates a long running query when adding the grades and modified grades, along with history.  The query uses left joins on both the grade and grade history table, however, it also excludes records records that are not in those tables in the final query.  Changing these left joins to inner joins makes the query run quickly, even on tables with 2+ million records. The code can be found in <MOODLE>/grade/classes/privacy/provider.php lines 207-224.  The proposed fix changes this code to 

 $sql = "
SELECT DISTINCT ctx.id
FROM {grade_items} gi
JOIN {context} ctx
ON ctx.instanceid = gi.courseid
AND ctx.contextlevel = :courselevel
JOIN (
SELECT a.id, a.itemid FROM {grade_grades} a WHERE a.userid = :userid1 OR a.usermodified = :userid2
UNION
SELECT b.id, b.itemid FROM {grade_grades_history} b WHERE b.userid = :userid3 OR b.loggeduser = :userid4 OR b.usermodified = :userid5
) gg ON gi.id = gg.itemid";