post meta - What type of index should I use for postmeta?

I already added a composite index but it still took 5 seconds to finish these 2 queries.

SELECT COUNT( DISTINCT( wpp.id ) ) 
FROM `wpdgls_posts` wpp JOIN `wpdgls_postmeta` wpm ON wpp.id = wpm.post_id
WHERE wpm.meta_key = '_form_id' AND wpm.meta_value = '14' ;

SELECT COUNT( DISTINCT( wpp.id ) ) 
FROM `wpdgls_posts` wpp JOIN `wpdgls_postmeta` wpm ON   wpp.id = wpm.post_id
WHERE wpm.meta_key = '_form_id' AND wpm.meta_value = '14' ;

Here are my index:

KEY `post_id` (`post_id`),
KEY `meta_key` (`meta_key`(191)),
KEY `meta_value` (`meta_value`(255)),
KEY `wp_meta` (`post_id`,`meta_key`,`meta_value`(8))

Should I use FULLTEXT index on meta_value?