Wordpress performance issues Using temporary; Using filesort specific query

I'm having some performance issues by the way a query is run, and I'm not sure how to change it.

mysql> explain SELECT SQL_CALC_FOUND_ROWS  wp_posts.* FROM wp_posts  JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)  WHERE 1=1  AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'publish') AND wp_postmeta.meta_key = 'Thumbnail-150x75'  GROUP BY wp_posts.ID ORDER BY wp_posts.post_date DESC LIMIT 0, 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: wp_posts
         type: ref
possible_keys: PRIMARY,type_status_date
          key: type_status_date
      key_len: 124
          ref: const,const
         rows: 21058
        Extra: Using where; Using temporary; Using filesort
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: wp_postmeta
         type: ref
possible_keys: post_id,meta_key,meta_post_id
          key: post_id
      key_len: 8
          ref: mm.wp_posts.ID
         rows: 1
        Extra: Using where
2 rows in set (0.00 sec)

Code (markup):

Using temporary; Using filesort is the bad part

I want to use this:

mysql> explain SELECT wp_posts.* FROM wp_posts  JOIN wp_postmeta ON (wp_posts.ID = wp_postmeta.post_id)  WHERE 1=1  AND wp_posts.post_type = 'post' AND (wp_posts.post_status = 'publish') AND wp_postmeta.meta_key = 'Thumbnail-150x75' ORDER BY wp_posts.post_date DESC LIMIT 0, 5\G
*************************** 1. row ***************************
           id: 1
  select_type: SIMPLE
        table: wp_posts
         type: ref
possible_keys: PRIMARY,type_status_date
          key: type_status_date
      key_len: 124
          ref: const,const
         rows: 21058
        Extra: Using where
*************************** 2. row ***************************
           id: 1
  select_type: SIMPLE
        table: wp_postmeta
         type: ref
possible_keys: post_id,meta_key,meta_post_id
          key: post_id
      key_len: 8
          ref: mm.wp_posts.ID
         rows: 1
        Extra: Using where
2 rows in set (0.00 sec)

Code (markup):

Any suggestions?