SELECT 
  pfv.variant_id, 
  pfv.position, 
  pfvd.variant 
FROM 
  cscart_product_feature_variants AS pfv 
  INNER JOIN cscart_product_feature_variant_descriptions AS pfvd ON pfv.variant_id = pfvd.variant_id 
  AND pfvd.lang_code = 'en' 
WHERE 
  pfv.variant_id IN (
    6316, 6315, 6320, 6321, 6418, 6419, 6420, 
    7164, 6331, 6454, 6453, 6330, 6329, 
    6505, 6501, 6504, 6506, 6475, 6476, 
    6477, 6478, 6479, 6480, 6481, 7168, 
    6427, 6426, 6425, 6428, 6424
  )

Query time 0.00088

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "pfvd",
      "access_type": "range",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "9",
      "used_key_parts": ["variant_id", "lang_code"],
      "rows": 30,
      "filtered": 76.667,
      "index_condition": "pfvd.variant_id in (6316,6315,6320,6321,6418,6419,6420,7164,6331,6454,6453,6330,6329,6505,6501,6504,6506,6475,6476,6477,6478,6479,6480,6481,7168,6427,6426,6425,6428,6424) and pfvd.lang_code = 'en'"
    },
    "table": {
      "table_name": "pfv",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["variant_id"],
      "ref": ["herbaria_cscart_4_18.pfvd.variant_id"],
      "rows": 1,
      "filtered": 100
    }
  }
}