In the developer forum (at http://moodle.org/mod/forum/discuss.php?d=195237) I suggested a rewrite of one of the SQL queries involved in the upgrade to the new quiz engine:

Replace the following code (from question/engine/upgrade/upgradelib.php):

$questionsstatesrs = $DB->get_recordset_sql("
SELECT *
FROM

{question_states}
WHERE attempt IN (
SELECT uniqueid FROM {quiz_attempts} WHERE $where)
ORDER BY attempt, question, seq_number, id
", $params);

with this

$questionsstatesrs = $DB->get_recordset_sql("
SELECT DISTINCT s.*
FROM {question_states}

s
JOIN

{quiz_attempts}

ON (s.attempt = uniqueid)
WHERE $where
ORDER BY s.attempt, question, seq_number, s.id
", $params);

It seems to be equivalent and is faster.