The retrieval of this list can be quite slow in a tracker with a consequent number of artifacts. When you have a filter on your tracker report on e.g. "Submitted By" it starts to be an issue.

The query to retrieve the list of submitter (the one to retrieve the list artifact updaters is similar) looks like this:

SELECT DISTINCT user.user_id, CONCAT(user.realname, '(', user.user_name, ')') AS full_name, user.realname
FROM tracker_artifact AS a
INNER JOIN user ON (user.user_id = a.submitted_by AND a.tracker_id = ?)
ORDER BY user.realname

which leads to this execution plan:

+----+-------------+-------+------------+--------+-----------------------+----------------+---------+-----------------------+------+----------+---------------------------------+
| id | select_type | table | partitions | type   | possible_keys         | key            | key_len | ref                   | rows | filtered | Extra                           |
+----+-------------+-------+------------+--------+-----------------------+----------------+---------+-----------------------+------+----------+---------------------------------+
|  1 | SIMPLE      | a     | NULL       | ref    | idx_tracker_id,idx_my | idx_tracker_id | 4       | const                 | 7041 |   100.00 | Using temporary; Using filesort |
|  1 | SIMPLE      | user  | NULL       | eq_ref | PRIMARY               | PRIMARY        | 4       | tuleap.a.submitted_by |    1 |   100.00 | NULL                            |
+----+-------------+-------+------------+--------+-----------------------+----------------+---------+-----------------------+------+----------+---------------------------------+

Notice the large number of rows that needs to filtered to deal with the duplication/ordering/concatenation.

By moving the de-duplication earlier in the query we can help the DB engine:

SELECT user.user_id, CONCAT(user.realname, '(', user.user_name, ')') AS full_name, user.realname
FROM user
JOIN (
    SELECT DISTINCT tracker_artifact.submitted_by FROM tracker_artifact WHERE tracker_id = XXX
) AS tracker_submitted_by ON (user.user_id = tracker_submitted_by.submitted_by)
ORDER BY user.realname

Execution plan of the modified query:

+----+-------------+------------------+------------+--------+-----------------------+---------+---------+-----------------------------------+------+----------+---------------------------------------+
| id | select_type | table            | partitions | type   | possible_keys         | key     | key_len | ref                               | rows | filtered | Extra                                 |
+----+-------------+------------------+------------+--------+-----------------------+---------+---------+-----------------------------------+------+----------+---------------------------------------+
|  1 | PRIMARY     | <derived2>       | NULL       | ALL    | NULL                  | NULL    | NULL    | NULL                              |  441 |   100.00 | Using temporary; Using filesort       |
|  1 | PRIMARY     | user             | NULL       | eq_ref | PRIMARY               | PRIMARY | 4       | tracker_submitted_by.submitted_by |    1 |   100.00 | NULL                                  |
|  2 | DERIVED     | tracker_artifact | NULL       | range  | idx_tracker_id,idx_my | idx_my  | 8       | NULL                              |  441 |   100.00 | Using where; Using index for group-by |
+----+-------------+------------------+------------+--------+-----------------------+---------+---------+-----------------------------------+------+----------+---------------------------------------+

On my test instance this gives an interesting performance improvement: the initial query takes ~0.27sec, the modified one ~0.02sec.