SELECT 
  post_id 
FROM 
  cscart_dosiin_posts 
ORDER BY 
  post_id DESC 
LIMIT 
  5, 20

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_dosiin_posts",
          "access_type": "index",
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["post_id"],
          "rows": 25,
          "filtered": 100,
          "using_index": true
        }
      }
    ]
  }
}