There is a significant performance bottle neck  in the compute function in grade_items 

https://github.com/moodle/moodle/blob/2cea0bd6c66300813457a2f18dc9a4a075efbc93/lib/grade/grade_item.php#L2119

The query looks like this

$sql = "SELECT $fields
FROM {grade_grades} g, {grade_items} gi
WHERE gi.id = g.itemid AND gi.id $usql $usersql AND gi.courseid=?
ORDER BY g.userid";

and $usql contains a list of gradeitems

No gradeitem fields are returned from the query and there appears to be no benefit from filtering by gi.courseid because
we are already filtering by grade_items - the method that builds the list of grade_items ("depends_on") already
filters the list of grade items to a specific course.

If the query is reformulated as:

$sql = "SELECT $fields
FROM {grade_grades} g
WHERE g.itemid $usql $usersql
ORDER BY g.userid";

then the execution time drops by a huge amount. This was discovered when calling the 
quiz_delete_attempt($attempt, $quiz);
In a utility and the performance made it unusable. 

In grade_category.php we see exactly the same issue with the following SQL:

$sql = "SELECT $fields
FROM {grade_grades} g, {grade_items} gi
WHERE gi.id = g.itemid AND gi.id $usql $usersql
ORDER BY g.userid";

Another optimisation - in grade_category.php, the generate_grades method makes a call to
get grade items from a specific set of ids:

$sql = "SELECT *
FROM {grade_items}
WHERE id $usql";
$items = $DB->get_records_sql($sql, $params);
foreach ($items as $id => $item)

The function quiz_delete_previews in mod/quiz/locallib.php calls quiz_delete_attempt in a
loop.
Since quiz_delete_attempt ultimately results in the 'generate_grades' method being called
there is a risk of the same DB query being made repeatedly - this can be fixed using
static caching where the ids haven't changed from the previous call.

Outcome

These optimisations were tested against a system with 2.5 million grade_grades and 648 grade_items. Deleting attempts was exceptionally slow, and caused timeouts on machines with low ram (4Gb). Deletion can be done via

/mod/quiz/report.php?id=5&mode=responses

These optimisations reduced the time from around 90 seconds to under 10 seconds.