Active Tickets, which I have participated. (Slow)

gistfile1.txt

SELECT p.value AS __color__,
       t.id AS ticket, summary, component,
       priority, FROM_UNIXTIME(CONVERT(c.value / 1000000, UNSIGNED INTEGER), '%Y-%m-%d') AS due_date,
       t.changetime AS _changetime, description AS _description,
       reporter
  FROM ticket t
  LEFT JOIN enum p ON p.name = t.priority AND p.type = 'priority'
  LEFT OUTER JOIN ticket_custom c ON t.id = c.ticket AND c.name = 'due_date'
  LEFT JOIN (SELECT ticket AS id, author, MAX(time) FROM ticket_change WHERE author = $USER GROUP BY ticket) ticket_change USING (id)
  WHERE t.status <> 'closed' AND (owner = $USER OR author = $USER)
  ORDER BY CAST(p.value AS signed), c.value, t.changetime