Forgot to paste in the "explain" command from mysql:
# id, select_type, table, type, possible_keys, key, key_len, ref, rows, Extra
'1', 'PRIMARY', '<derived2>', 'ALL', NULL, NULL, NULL, NULL, '1351228', 'Using where; Using temporary; Using filesort'
'1', 'PRIMARY', 'priority', 'eq_ref', 'PRIMARY', 'PRIMARY', '1000', 'const,t.priority', '1', ''
'1', 'PRIMARY', 'milestone', 'eq_ref', 'PRIMARY', 'PRIMARY', '767', 't.milestone', '1', ''
'2', 'DERIVED', 't', 'ALL', NULL, NULL, NULL, NULL, '890440', ''
'3', 'DEPENDENT SUBQUERY', 'c', 'eq_ref', 'PRIMARY', 'PRIMARY', '504', '
trac.t.id', '1', 'Using where'