hello i'm having a strange execution time behavior over almost same sql queries

q1:

SELECT `t1`.`id`, `t1`.`key`, `t1`.`module`, `t2`.`value` 
FROM `translates` AS `t1` 
LEFT JOIN `translates_i18n` AS `t2` ON (`t2`.`id` = `t1`.`id` AND `t2`.`culture` = 'en') 
WHERE `t1`.`module` IN ('GLOBAL','AJAX','FORMS', .... about 15 items) LIMIT 9000;

0.10 sec

q2:

SELECT `t1`.`id`, `t1`.`key`, `t2`.`value` 
FROM `translates` AS `t1` 
LEFT JOIN `translates_i18n` AS `t2` ON (`t2`.`id` = `t1`.`id` AND `t2`.`culture` = 'en') 
WHERE `t1`.`module` IN ('GLOBAL','AJAX','FORMS', .... about 15 items) LIMIT 9000;

0.000... sec

tables definition:

CREATE TABLE IF NOT EXISTS `translates` (
  `id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `key` varchar(255) NOT NULL,
  `module` varchar(255) NOT NULL,
  `system` tinyint(3) unsigned NOT NULL DEFAULT '0',
  PRIMARY KEY (`id`),
  UNIQUE KEY `key` (`key`,`module`),
  KEY `module` (`module`)
) ENGINE=InnoDB  DEFAULT CHARSET=utf8 ;

CREATE TABLE IF NOT EXISTS `translates_i18n` (
  `id` int(11) unsigned NOT NULL,
  `culture` varchar(2) NOT NULL,
  `value` text NOT NULL,
  PRIMARY KEY (`id`,`culture`),
  KEY `culture` (`culture`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;


ALTER TABLE `translates_i18n`
  ADD CONSTRAINT `translates_i18n_ibfk_1` FOREIGN KEY (`id`) REFERENCES `translates` (`id`) ON DELETE CASCADE ON UPDATE CASCADE;

difference between q1 & q2 is in t1.module column, that is also in where statement

i just cant see where is the problem, please, anyone poit me

update

q1:

mysql> EXPLAIN SELECT SQL_NO_CACHE  `t1`.`id` ,  `t1`.`key` ,  `t1`.`module` ,  `t2`.`value` 
    -> FROM  `translates` AS  `t1` 
    -> LEFT JOIN  `translates_i18n` AS  `t2` ON (  `t2`.`id` =  `t1`.`id` 
    -> AND  `t2`.`culture` =  'en' ) 
    -> WHERE  `t1`.`module` 
    -> IN (
    -> 'GLOBAL',  'AJAX',  'FORMS',  'ROOTMENU',  'LANGSWITCHER',  'AUTHORIZATION',  'MENU',  'MINIFY',  'SIMPLESHOP'
    -> )
    -> LIMIT 100500
    -> ;
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
| id | select_type | table | type   | possible_keys   | key     | key_len | ref                 | rows | Extra                    |
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index  | module          | key     | 1534    | NULL                |  627 | Using where; Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY,culture | PRIMARY | 12      | theloom.t1.id,const |    1 |                          |
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
2 rows in set (0.00 sec)

q2:

mysql> EXPLAIN SELECT SQL_NO_CACHE  `t1`.`id` ,  `t1`.`key` ,  `t2`.`value` 
    -> FROM  `translates` AS  `t1` 
    -> LEFT JOIN  `translates_i18n` AS  `t2` ON (  `t2`.`id` =  `t1`.`id` 
    -> AND  `t2`.`culture` =  'en' ) 
    -> WHERE  `t1`.`module` 
            -> IN (
    -> 'GLOBAL',  'AJAX',  'FORMS',  'ROOTMENU',  'LANGSWITCHER',  'AUTHORIZATION',  'MENU',  'MINIFY',  'SIMPLESHOP'
    -> )
    -> LIMIT 100500;
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
| id | select_type | table | type   | possible_keys   | key     | key_len | ref                 | rows | Extra                    |
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
|  1 | SIMPLE      | t1    | index  | module          | key     | 1534    | NULL                |  627 | Using where; Using index |
|  1 | SIMPLE      | t2    | eq_ref | PRIMARY,culture | PRIMARY | 12      | theloom.t1.id,const |    1 |                          |
+----+-------------+-------+--------+-----------------+---------+---------+---------------------+------+--------------------------+
2 rows in set (0.00 sec)