SELECT 
  COUNT(post_id) 
FROM 
  cscart_discussion_posts 
WHERE 
  thread_id = 13697

Query time 0.00045

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_discussion_posts",
          "access_type": "ref",
          "possible_keys": ["thread_id", "thread_id_2"],
          "key": "thread_id_2",
          "key_length": "3",
          "used_key_parts": ["thread_id"],
          "ref": ["const"],
          "rows": 2,
          "filtered": 100
        }
      }
    ]
  }
}